I've never entered my real birth date with those age gates. I always just choose the year, scroll down to some random number (which is usually somewhere in the '50s) and click enter.
It's obvious the age gates are only there so that if parents start complaining they can point at the age gate saying "we've already put warnings, and a test there. We've done the best we can do. Don't blame us". Quite sad that they have to do this, since the problem is the fact that parents don't pay attention, just like you pointed out in the article.
Then again, I remember my mom helping me by answering the questions so I could play the first Leisure Suit Larry game when I was 10. She knew what she was allowing me to play as well, but deemed me intelligent and adult enough to handle a game like that at my age.
In fact, she'd watch me play the game because she thought it was a funny game herself, but she had no interest in playing a point n click game.
Same with how she watched me play the Discworld, Monkey Island and even Metal Gear games.
I've always wondered about the age gates... I remember seeing them with on the Angry Video Game Nerd, which made me chuckle. Most of his viewers are under 18, so if the age gate would have worked, he'd lose all of his viewers.