But remember that it had a decent third-act twist: the prehistoric world of the film was actually a post-nuclear holocaust realm. In short, it ended like Planet of the Apes.
Vaughn obviously didn’t look teen-aged in the film. He was around 25 when it was shot, and looked 29 or 30. He would’ve been convincing if the film had been titled 20something Caveman With a Mortgage and a Baby On The Way.