I may have an extremely warped opinion on this due to several reasons (imo mostly due to irl encounters with adult people that would put you on a watch list due to how young they look) but I think in the end it usually boils down to anime/manga just being terrible media for portraying how old a character actually is. The oldest anime character you can draw will still look significantly younger than a person you meet irl simply because the art style hides a lot of the age marks.

Edit: which is not to say that there isn't a lot of CSAM hiding, it's just imo the stuff that gets popular on SFW platforms is rarely that stuff