Hinduism is often said to be one of the oldest religions of the world. It is definitely the oldest among the living religions. How old is it? How can you say an age for a religion which can not be defined? As Buddha said once in his dialogue. "You cannot talk of a way, unless you have a destination? It is like building a stairway to a house, which do not exist."

Is Hinduism a Religion?
Truly speaking, when we talk of Hinduism, we do not know whether we are talking about a religion, or a group of religions, or a group of beliefs and traditions (some of which are diametrically opposed to each other.) This some times give rise to the argument that Hinduism cannot be considered as a religion in the strictest sense of the word, but as a concept that incorporates into itself many religious thoughts and world views. 

What do we mean by the word "Hindu" and "Hinduism"?
Various origins for the word "Hinduism" have been suggested:
It may be derived from an ancient inscription translated as: "The country lying between the Himalayan mountain and Bindu Sarovara is known as Hindusthan by combination of the first letter 'hi' of 'Himalaya' and the last compound letter 'ndu' of the word `Bindu.'" Bindu Sarovara is called the Cape Comorin sea in modern times

It may be derived from the Persian word for Indian.
It may be a Persian corruption of the word Sindhu (the river Indus)
In all these cases it referred to a gegraphical region and not to a religion. This is simply because there never had been a single unified relgion in India. Inspite all the claims of Sanadhana Dharma and eternal religon, there never existed such a religion. Look what major Hindu organisations state. These are collected from well known Hindu Religous sites.

Hinduism differs from other organized religions in the following aspects:

It is not based upon a particular founder.

It is not based upon a particular book.

It is not controlled by a central institution or authority such as a church or a sangha or association.

It is not averse to examine and assimilate fundamentally diverse thoughts and beliefs into its system.

It accepts other religions as various paths to salvation and does not favor organized attempts to proselytize people.

It has been evolving continuously, through internal reforms and as a  reaction to the threats and challenges without. 

Once this fact is established the rest would be easy to discern.
Since it has no authority in any form of person, documentation, or institution it simply means anything can form part of this religion. In actual fact this has been what was going on in history. Yet persons have been brought in as authority and Vedas and other forms of documentation are always quoted as proof. Like any other religion each group of interpretation form their own niche of institutions and as time goes on they are being multiplied. They quote documents such as Vedas and Upanishads and Puranas as though they are scriptures. A scripture has validity only in so far it has the backing of an absolute authority, which Hinduism on the outset refutes. So it will not be surprising that various proponents give various definitions of Hinduism to suite their thinking pattern.

If Hinduism is a religion of no scripture, no authority, and no institution! What then is it?
  It has even been suggested that the word Hinduism was invented by the British administration in India during colonial times. But the description of Hindava Samskara (Indian Civilization) had been in existence long before the British. But then it did not refer to a religion. Hindu as a name of the religion was actually invented by the British to refer to all Indian religons with all its diversities.. In their ignorance or for convenience they simply dumped all Indian religions together as Hinduism. As you will notice from what we discuss below. There is no such religion called Hinduism. However because the British treated all Indian religons as Hinduism, it created a common front in spite of their opposing theologies. As the British rule proceded even Budhism and Jainism and Sikhism were considered part of Hinduism even though they were the result of intense opposition to the Brahminic and Aryan Religions. It was the 250 years of British rule that produce the amorphous religon of Hinduism. The resultant education even brain washed the Indians. This was given an acceleration under theosophy which was an attempt to form a syncretic universal religious form. In the Indian Independence Movement this unification of religion became very important. Annie Besant and Gandhi used this as a powerful tool to integrate all India under a psuedo religon. Gandhi even tried to incorporate Islam and Christianity into it. This is what enbittered the rest of the Aryan religons. The assimiliation of Dravidian and other relgions into Aryan Religion was a worthwhile move. But Islam and Christianity posed a threat to the conquest. Thus what is today known as Hinduism is a product of the twentieth centuary.

The change took place in less than a centuary. When I was a boy I was given the ten incarnations of Vshnu in the sloka:
Matsys, Kurma, Varahancha, Narasimhacha, Vamana, Ramo, Rama, Ramacha, Krishna, Kalki Janardhana. But in this study I have been searching for the ten avatars and was surprised to see that balabhadra Rama was simply displaced by none other than Budha. This is simply the direct evidence of the evolution of Hinduism as we see and know today..After half centurary of learning Hinduism when I visited Madurai in Tamil Nadu, when the priest in temple presented to me the ancient Sivite theology of Formelss Form, I could not believe it. I had to reconfirm its validity with my friends in the theological college.
"Hinduism as a faith is vague, amo rphous, many sided, all things to all men. It is hardly possible to define it, or indeed to say definitely whether it is a religion or not, in the usual sense of the word, in its present form, and even in the past, it embraces many beliefs and practices, from the highest to the lowest, often opposed to or contradicting each other."

Jawaharlal Nehru, `The Discovery of India', John Day, 1946, p.66.
"Hinduism as defined in contemporary parlance is a collation of beliefs, rites and practices consciously selected from those of the past, interpreted in contemporary idiom in last couple of centuries and the selection conditioned by historical circumstances ... in a strict sense, a reference to `Hinduism' would require a more precise definition of the particular variety referred to Brahminism, Brahmo-Samaj, Arya Samaj, Shaiva Siddhanta, Bhakti, Tantricism or whatever."
Todays Hinduism is continuation of contradicting traditions:
" May noble thoughts come to us from every direction. " Rg Veda
Ability to absorb noble thoughts as well as ignoble thoughts - ideas that make sense and some that do not make sense - and produce a web of intricacy was one of the greatest strengths of Hinduism of this centuary. Syncretism is its strength. Ability to use these to the benefit of the few in the dominant class was its curse. This is inevitable in a fallen society where an eternal class war is ever present. Where "Self" is all there is and where "I am God" there cannot be another way. This embedded class war is expressed in their concrete form in war between families, tribes and nations. It has a double edge and cuts both ways. So Bhagawat Gita with its sublime teachings of ways of salvation is placed in a context which essentially teaches that "Killing is OK if the opponent is usurping your rights" While Ahimasa is the catchword of modern Hinduism, Killing ones brothers is made sacred in Bhagawat Gita. Only the most exquisite religion like Hinduism alone can achieve that. How one can be blinded by the lofty philosophy to hoodwink this I still cannot fathom. Even if the whole Mahabaharatha is an Aryan Tale, its teaching cannot be read otherwise.

Hinduism is not a monolithic Asvaththa Tree but a forest.
We cannot therefore define Hinduism in any structured way. It is not one monolithic tree whose roots are above and whose branches are spread throughout below. It is rather a forest filled with a million trees each having its own branches and roots and is still growing - both in number of trees and in their branches! The branches are the various new schools of thought and practices that were incorporated into it during the subsequent periods in history. It would be presumptuous to define that the trunk is belief in the eternal nature of soul and of supreme God because there are other trees in that forest, which are atheistic and deny even the existence of any God. But in defining Hinduism, most theistic Hindus assume God or gods in some way; but none agreeing with each other. But it is important to understand that the great ninth avatar of the Hindu God - Buddha - himself denied the existence of God. Was this avatar ignorant of what he really was? Hinduism became more or less like an ocean that would absorb every thing that flowed into it from all directions. These numerous streams brought in a rich variety of minerals.
Thus the modern Hinduism is never static. It evolved continuously from stage to stage; society to society; and went on transforming itself continuously. This was due to the contribution by  many scholars, seers, sages, institutions, kings and emperors, priests, prophets, rebels, interpreters, and commoners over a vast period of time. Evidently reason or sanctity did not always lead it (because sanctity cannot be defined independent of an unchanging God). Mostly it was evolved by the dominant class of each period. By interpreting, deviating, changing, correcting, molding, manipulating, modifying, and integrating various aspects -of religion and even god - to suit the social, political, material, intellectual context of the time. These men - great, saintly,but often scheming and cunning - built up a formidable system. This conclusion is unavoidable. History bears witness to this conclusion.

Very ordinary people living in remote villages, the working  classes, the illiterate peasants and laborers also contributed to the development of Hinduism as much as elite and intellectual upper castes. They developed elaborate myths demi-gods, spirits, plants, snakes, lakes and rivers and introduced rituals and festivals which some times included blood thirsty gods who demanded animal and human sacrifices. They too formed part of the society and heritage and they also slowly became integrated into Hinduism. In fact even today in every village they have their own village gods, which are unfamiliar to those outside. Hindu elite used these myths and legends also to their advantage and built over it an elegant imposing structure.
In all these evolutionary process there is no doubt who triggers and guide the process. It has always been guided by the dominant class in order to subjugate the others. It is the Aryans - the Noble Ones - that always win. Now they can even claim the ancient Indus valley civilizations even as their own!! The British ignorance suited the dominant class and they evolved a religon out of it.

Please. Let us not forget that in a forest there are lot more beautiful trees than the tallest ones.
How Old is Hinduism?
If we accept this truth, - and there seems to be no reason why we should not, the question is irrelevant. How can you ask the age of a religon that do not exist? If we are talking about any religon as Hindu religion it started in the prehistoric days, when the aboriginal men were slowly trying to understand the mysteries of their  own existence and the mysteries of the world around them and were trying to build a stable society. It was founded on the survival of the fittest. When a Hindu talks about Sanadhana Dharma (Eternal Religion) he actually means this - a religion which is so amorphous and undefined without form and consistent content. And it is true. this religion of Self - started with the beginning of man.

Theistic Hinduism
It is therefore important that we make sure to categorize the Hindu religious groups. We will talk hence forth about the "Theistic Hindu Forest" avoiding the clutter of other traditions, though it is often mixed up and impossible to isolate. One problem talking to my Hindu brothers was their ability to jump from one tree to another as occasion demand. So we need to concentrate and address those groups who has authority, who has a well defined scripture and who has norms and fixed traditions to get anywhere. The normal Hindu will hold on to it only so far it can be held without problem. Once challenged they will leave it and embrace another. So it will be a futile attempt of chasing after the shadows. In the other articles I have tried to interpret and understand the special interpretations of Hinduism by various intellectuals giants. But there are many which goes deeper into Abhichara witchcraft and god manipulation by mantras and tantras which are not commonly dealt with. These are based on the realism that spirits are not divine beings but are like humans essentially fallen. By suitable means they too can be subjected to serve the priesthood of man. Once subjugated they too will become slaves to the person and can be exploited. But they need to be kept constantly in chains. There is a whole realm of spirits in existence in the created universe. Among them are fallen races like man.