Why are all super-heroes in the 31st century teenagers?

Not all of the 31st century heroes are teens (in fact, I'm not sure if there's any teen at all in the currently spotlighted Legion).

Maybe in the future, older superheroes retire early. But yeah good point. The legion started as a clubhouse of teens with unique abilities. And then they got big, the galactic federation known as the United Planets sanction them as a peacekeeping force. The only recognized group of heroes. You also have the Wanderers, the heroes of Lallor, but the Legion of Superheroes was like the Jedi in Star Wars. They are in the big leagues.

Later storylines, the characters have aged into twenty somethings, and have admitted older members. But yeah, still a good point, why are there no older Superheroes?

Can someone explain how Booster Gold has a ring but is from an earlier time?