There are many websites that believe I was born on January 1st, in a year close to my actual birth year.
When it's actually required by some law or regulation (e.g. financial stuff) I give my actual birthday. But when some site is just wanting to comply with age verification? Yep, I'm over 30, so you don't need to see my identification. (Jedi hand wave).
Well, they would have the legal right to force-choke your account, or chain your partner to a golden bikini, when they discover that you weren't abiding by the Terms and Conditions which you agreed to. Seems fair.