... What about Jay Garrick? He's arguably improved as a character because of what he represents, the old guard who inspired the current generation of heroes. And he started out at the same age as Batman and Superman.
Fictional characters should be allowed to change, if it improves their stories. Peter going to college, losing his girlfriend, struggling to balance gets ends to meet with his actions as Spider-Man, all arguably helped give Peter his everyman status that made him so popular, and made it feel like his character was growing. There are limits of course, but that's true of every hero.