My understanding of "prehistoric" events was things that happened to early to survive in recorded history. So, Adam and Eve wouldn't be prehistoric because, assuming they actually existed, the knowledge was preserved long enough to be recorded.
According to google, however, the word usually means things that happened before the development of writing. That seems odd to me, given my point about ancient histories almost never being contemporaneous accounts, but whatever.
Also, the Iliad was thought of as being more or less historical for a long, long time. Plus, the timeframe it supposedly happened was definitely after writing had been developed in Greece, though, IIRC, there are no examples of it from the Greek "dark ages" around when the Trojan war was supposed to have happened.