Anime and Youth Culture

I am not sure if this has been discussed before but what is it with the Japanese obsession with youth? I know this isn't limited to Japan but it seems as though with any character who is either in their 20s is considered to be an old man age wise despite the fact that there are still young (sort of). I know this may answer my question but I found this on a reddit:

Quote:

That has much more to do with the Japanese idea that since one must work endlessly as an adult they should get all their adventuring out of the way as a child.

This sort of explains a lot of things in anime but still why is 20s and 30s considered to be old despite nearing only quarterlife?

Anime and Youth Culture