'Lockwood & Co." on Netflix is a good example of this idea. Only a small number of teens can fight the baddies.
'Buffy the Vampire Slayer' assumes that the Slayer won't make it past 25...
Can't think of that many more.
I was thinking about this a while ago. Back in the day, there were a lot of middle-aged superheroes. Dr. Strange, Col. Nick Fury, Reed Richards, Professor X, and The Chief of the Doom Patrol were all over 40.
I can do two hours on the Caped Crusader and still have material left over...
The Batman animated version is supposed to be him in Year One, about 25 years old. In Batman Beyond he's supposed to be over 70. According to the last DC comic's fact sheet, they are currently portraying him as having been active for about 15 years.