Part I of a Feature on GTA V that focuses on the terrible truths of Los Santos:
Then I noticed that none of the damn characters -- playable, NPCs -- ever put their seatbelts on. So besides wildly out-of-control crime in Los Santos, every single car owner is living in the Danger Zone all the time!
You'd think they'd actually want to be more careful living in a world where the human population is in permanent decline.
GTA V appears to indicate that children are (or at least, have been) born and physically grow up. There's some evidence of that with Michael's kids, but one never sees a person under the age of about 18 or 19. Something happened to the children or something rendered the entire populace infertile and no one's talking about it.