That is not "required by US federal law". I assume you're confusing with COPPA, which requires verified parental consent for a user under the age of 13 to register, if the platform performs data collection on users for commercial use or advertising. The platform could very easily limit registration to a much higher age if they so desired, and that wouldn't be 'illegal'. It's the pairing of allowing young users to register on a platform that could feature 'sex-positive' content (intentionally or mistakenly) is why people are criticizing it.