It's not required by the EU. If not all the sites that have content not appropriate for children, such as porn sites, or Reddit, would need to implement this. And of course you wouldn't give your ID or credit card to a porn site.
Google is the only company that started to implement this. To me it's not required by law but they decided to do so to get more user data. Also because having a credit card doesn't confirm nothing about your age. I use a credit card to buy things online since I'm 13, you can have a credit card even if you are not 18, simply your parents need to sign for it, but then it can have your name on it.
In 2010, Google required your ID card if you enter a false birth date, and you are locked out of your account in the meantime. Happened to a colleague of mine because we were using Google Apps for Business. Because he didn’t want to give his real info to Google at work, he couldn’t work for 2 days and had to give Google his ID.
> if they know it's false why do they ask for it, they already have the right one
Great question. It seems like it's being used as authentication, rather than authorization, in that it's meant to prove that you are who you say you are.
An alternative would be entering your passport or something which would be even crazier