Nobody is trying to age restrict the safari app (like a movie theater), but I don't see why websites or parts of websites can't be restricted like individual movies are as they are given a rating by an authority.
But other apps entirely like dating apps? or only fans? can probably be entirely restricted to some age.
This isn't even a hard question. The movie theater is open but movies that are rated R are not. In this case, Reddit.com is a movie theater, subreddits are movies. The website might be open, but not every subreddit is. This is in fact how Reddit already operates, age verification is just a joke right now.
But other apps entirely like dating apps? or only fans? can probably be entirely restricted to some age.