Hinduism is known as the worlds oldest organized religion.There is no specific year that it was started on.
Hinduism cannot be described as an organized religion. It is not founded by any individual. Hinduism is God centred and therefore one can call Hinduism as founded by God, because the answer to the question 'Who is behind the eternal principles and who makes them work?' will have to be 'Cosmic power, Divine power, God'
Hinduism is not one religion, but a mix of several religions, one added on top of another. So what answer can one give if he or she is asked on when Hinduism started. There was a time about twenty thousand years ago, when the whole world was covered with snow and become uninhabitable; and our forefathers lived in the arctic region. In the early part of the twentieth century, Balgangadhar Tilak, a prominent Indian freedom fighter, was jailed in his struggle for Indian Independence . He spent his time at the jail by reading Rig Veda. And, while doing so, he noticed something very peculiar about some of the verses. He noticed that some verses of Rig Veda were referring to what looked like the region about the Arctic ice cap, north of Europe. He published this in a book titled The Arctic Home in the Vedas, in 1903. Tilak wrote -
"There are many passages in the Rig-Veda, which, though hitherto looked
upon as obscure and unintelligible, do, when interpreted in the light of recent
scientific researches, plainly disclose the Polar attributes of the Vedic dieties,
or the traces of an ancient Arctic calendar; while the Avesta expressly tells us
that the happy land of Airyana Vaêjo, or the Aryan Paradise, was located in a
region where the sun shone but once a year"
So the date of Hinduism predates the last ice age atleast. However, if you want to think about the Hindu Trinity that Hindus worship today, the trinity was not part of the religion at that point of time - so, in that sense Hinduism postdates the last ice age - whichever you take it, based on what exactly you mean by the word Hinduism.
(Excerpts from book "19000 Years of World History")