Robert Vaughn in Teenage Caveman (1958)
That's hardly a knock against post-collapse settings. The concept is as solid as they come. There's a good reason it's all over Appendix N. But it's interesting to think what the opposite would mean for a game. I don't recall too many RPG settings taking place in young or pseudo-prehistoric worlds (though the Lost World trope is fairly well represented). Is there a Hok the Mighty RPG? Fire & Ice? It seems like a missed opportunity. What's not to love about playing a Primitive Savage fighting in a world ruled by thunder lizards? More interesting are the stakes of your survival, which may also mean the survival of your tribe, even your entire species.
Seems like you could do a kind of Sid Meier's Civilization take on the D&D endgame, where instead of building a stronghold and attracting followers at 9th Level you're growing and advancing your tribe from your very first adventure. Experience points could be cashed in to raise tribal stats like Culture, Resources, Military Strength, etc. Almost like Settlers of Catan, but instead of drawing for resources you go on adventures to get them. Grab your obsidian axe, Grodd. Tonight we steal sheep!
Tomorrow: Serpent Men and the Hollow Earth
EDIT: There's something in the water, because Jeff is talking about this today too.