Many users do not desire to interact with underage users for many reasons. Some underage users may wish to opt out of seeing 18+ users so they may bond with people closer to them in age or perhaps for safety. I think a functionality to
- mark your account as 18+
- hide content from accounts who didn't mark self as 18+
- hide content from accounts who marked self as 18+ would be very useful to those users and set a higher standard of personalized experience for users engaging with social media.
You don't need users to prove their age for it to be effective. Some people will lie, but the majority will probably tell the truth. You'll probably want stronger protections for something like a dating site, but I can still see value in something like Lemmy.