I ususally give my correct age (21 in 10 days so who cares), but I often just wanted to enter an improbable age, like 148 or something, just for giggles...or 2 years...
Well, this is not to lock minors out of porn, it is a liability for the site owner.
Not having such a system would make the website "have explicit content ready for minors", but having such a system puts them into the position of "Hey, if your kid lied to see the porn and voilence, it is not our fault, is it?", and thus their lawyers have something to weight against the accusation.
It is a legal liability, it is not actually meant to shield anything from anyone.
Altho, if your children are honest enough to enter their real age, you, as a parent, can be proud.