I think one issue with this is that there is no way to really rigorously define the question. If I were to come up with an equivalent, it would be like asking a biologist what the oldest kind of turtle is--sure, you can define the terms such that you can sort of get an answer, but ultimately every type of turtle is just as much a continuation of other types of turtle, and furthermore it is functionally impossible to draw a hard line between one type of turtle and the type of turtle it came from. Ultimately, no matter how far you delve with this, it will just be turtles all the way down.

Anyway, the oldest "sacred text" in use is the Rigveda, the oldest part of the Vedas, which is probably somewhere between 3000 and 3500 years old, although credible estimates do sometimes push it a bit farther back than that. This compares pretty respectably with the Egyptian Book of the Dead, which is kind of still is use by New Age types, although the continuity break there is so severe I don't really count it. That aside, the Rigveda is a useful book to look at this through the turtle problem, because saying that it is yea old, therefore Hinduism is yea old opens yourself to some problems. If you think about stereotypes of Hindus, just as a basic level, one of the first ones will have to do with cows vis a vis Hindus not eating them, and yet the Rigveda explicitly talks about the sacrifice and eating of oxen. This is a bit cosmetic but it goes to show just how extensively "Hinduism" has changed over the millennia, and in general it did not begin to resemble modern Hinduism until the eighth century or so. So how old is Hinduism?

Not to discourage further discussion but a similar question has been answered before by /u/yodatsracist in What is the oldest religion of which we can explain the basic tenets?