I have been doing research on this, but I just can find a lot on the question of when Zoroaster [editor: the founder of Zoroastrianism, the chief religion in Persia from about 500 BC until the time of Islam] lived. I know that some claim that he must have lived 1500 BC because the language which the Avesta was compiled in dates back to that time. But still no one ones when he was born.
Yes, it is true that Zoroaster incorporated some earlier material into his teaching. Some early material in the Avesta may predate Zoroaster. That is consistent with what I already said. [editor: this is a follow-up to an earlier question about when Zoroaster lived in which I pointed out that he included aspects of an older religion endemic to Persia]
An analogous situation is in Confucianism. Confucius used much material which predated him by hundreds of years. The I-Ching has been estimated to have been written about 1000 BC, but Confucius lived more like 550 BC. This does not cause us to think that he lived before this material was written.
Similarly, Jesus used the Old Testament. This does not cause us to believe he lived 1500 BC.
Our sources on Zoroaster are much less reliable than those on Jesus (or Confucius, David or Isaiah or many other Old Testament figures, for that matter), so we do not know exactly when he was born and until what year he lived, but I have looked at the material more than once and fairly carefully, and all reliable sources say he probably lived some time in the seventh century, plus or minus one hundred years. MY tentative conclusion is that those who say he lived 1000 to 1500 BC are probably confusing the fact that some of the material in the Zoroastrian scripture may come from that early time–before he lived.