Buffy the Vampire Slayer.
This is one of the best-known examples, and includes the classic trope of "old guy has young person's tattoo." In the episode "Anne," Buffy runs away from home and goes to work as a waitress in a diner. But it turns out that young people are going missing in L.A., and then later they're turning up again — old and dead. Buffy discovers they're being sucked into Hell, where everybody ages more quickly. Contains the classic line: "Want to see my impression of Gandhi? Well, you know, if he was really pissed off."