I'm 41, but I still randomly fill that crap out because, as others have said, it's just too much damn trouble trying to scroll through everything to put my real birthdate.
Tho most of the time, I just have Replay Media Catcher running. Many of these sites start buffering the video behind the age-check, so it will latch on and record the whole video to your computer in far less time then it takes to watch without you having to click/enter anything. Bypass the age check, and can watch the video when I want with a REAL player (like VLC) with no pauses, skips, lag, etc.. Win-win there if you ask me.