Tis in their TOS that you have to be 18+ but you do not agree to the TOS until you create an account. You can browse without creating an account.

Same thing as posting a for sale ad in a storefront window with the contact information hidden. You have to enter the store and agree to their rules to get that contact info but if you just want to look...

FB has much greater control and can enforce such a rule, you've already agreed to their TOS which include giving them your birthdate.