That is not true. Either A) You know for a fact all content is safe for underaged, or B) You verify.

What is safe for underaged is not defined and can change on a whim. Therefor, any sane person running a website that is not "explicitly for underageds" will verify and eject said underageds. Especially since the one in charge (hired by the company) can be personally liable for any "harm" comming to the underaged.