YouTube just has nude yoga, Brazilian waxing, and other videos that are not age restricted.
Kids still use Youtube, though YouTube Kids exists and I don't know how much it's used.
Steam's already made that rule, anything you publish has to fit their card processor's criteria. Their written criteria is clearly not what they're using since legal things are what they oppose, so I guess you have to submit your game to all the card processors (they don't specify) so they can decide if it's acceptable. Hope they're hiring people for this.