I don’t think something like this is technically possible. Unless you have a policeman on entry checking their documents. If they can click “I’m 18+”, they can lie about anything. It’s their problem, not yours.
Most of “more than that” (e.g. forcing to bind account to a credit card, provide a verified documents photo, link to social media account) usually will harm valid users more than make the life a bit harder for liars/cheaters.
The only thing that can (and should) be done is to keep the cover art PG-13 as it may display along with other search results or in your profile.
I love the way Itch.io allows downloading games even to non-logged in users and not imposing dozens of limitations and privacy breaches on logged in ones, and I’d really like it to stay that way.