There is no "real" answer. Historians generally consider the fall of the Roman empire to be the dividing line between ancient and modern, but even that is not a hard line, but a rule of thumb. Prehistoric is considered to be before the advent of written history (originally first hand acounts) so you'd have to go back about 7,000 years or so.

Remembering that there are no hard and fast rules:
more than 5,000 B.C. is prehistoric
5,000 B.C. to 410 (or thereabouts) A.D. is ancient
410 A.D. to the present is modern