When did Google/YouTube start doing this, and why?

When did Google/YouTube start doing this, and why?
Now whenever I come across an "adult" (whatever the hell that means) video on YouTube, I have to provide proof of age. Is this a sick joke? I don't even have a credit card, but if I did I certainly would NOT give them my details!
Updates
+1 y
I keep forgetting that many people access GAG through their mobile phones, and don't use a desktop. Sorry for the small, unclear image :'(
Updates
+1 y
The image says:
Use a credit card to verify immediately
You won't be charged. Any transaction fee will be fully refunded.
Use your ID
Submit an image of a valid ID, like your drivers' licence or passport. It may take up to three days to verify your ID.
When did Google/YouTube start doing this, and why?
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