You have to have a credit card to get anywhere on Onlyfans, which automatically means users are reasonably validated as over 18. Performers get people to message them via Onlyfans or a similarly named site (also ends in fans but I won't name it) as reasonable proof that they are over 18, these two sites are held up as examples of pretty responsible sites overall.
Yes of course some kids will manage to put their parents' card details in but if the parents don't then go nuts when charges start appearing on their statement each month or the charges each time messages are sent via these sites, then to be honest all hope is lost anyway.
There is far far worse content openly accessible on ALL everyday sites including Twitter, Tiktok, Youtube, Snapchat and even Instagram if you start searching certain hashtags or find one adult account and then start looking up that account's followers etc, and none of that requires a credit card or any adult verification (Youtube requires to be logged in for certain videos but that's not difficult to achieve). Especially twitter, it's an utter cesspit. And that's before going anywhere near Pornhub or similar which are also openly viewable without any kind of account.
Anybody getting upset about Onlyfans is *incredibly* naiive about what's easily found on other sites.