In the Marvel universe, gamma radiation gives you super powers because the Celestials tinkered with human evolution. My favorite head canon for why no one ages in Marvel is the Franklin Richards doesn't want them to. When Marvel started in the sixties it ran in real time. Peter Parker was in high school for four years or whatever of our time, then went to college. No one knew they were creating cultural touchstones so this didn't seem like a bad idea. Franklin Richards is the most powerful earthbound being in Marvel. He can control reality. He was born in 1968 right around the introduction of "Marvel Time." The theory goes that Franklin doesn't want to grow up or for his friends to age and die so he has essentially stopped time for the entire universe. As for DC, there's a crisis that resets time like every two weeks.