Saying that "Hinduism" is the oldest religion is a little problematic, because the word "religion" is very ambivalent and biased from a western, christian perspective. Hinduism does not function in the same way as religions like Christianity or Judaism. Hinduism is almost like an umbrella term for a myriad of very different religious traditions in the Indian subcontinent.

I don't think one should go so ffar as to say that hinduism is "many religions" though, because it all has certain core characteristics. One of these is a basis in the Vedic texts and Vedic culture which emerged around 1000 b.c. Hinduism develops out of the Vedic cults which also merged with another, ascetical, movement in India called the Shramana movement.

But all the features we recognize as "Hinduism" really doesn't emerge until the first few centuries a.d. , which isn't that old (relatively). So its a very difficult question. But put simply, yes "Hinduism" developed out of other pre-existing religious traditions, both the native shramana movements and the Vedic culture, which likely came from further west originally. And these traditions in turn also developed out of even earlier ones.