Is it still though? Also, weren’t early super heroes mostly adult like Superman and Batman? I feel like back in the day when the audience were mostly children they used adult superheroes the kids could aspire to, then they started aging them down with Spider-Man to make them more relatable. Nowerdays the audience is mostly adult, maybe yearning for simpler times, certainly with a lot of nostalgia for what they used to see.

But to be honest I don’t think the premise is actually true. There are certainly some eternally young superheroes, but there are still and always have been lots of older superheroes.