Don’t get me wrong, I’m all for privacy. But between setting up the birthdate when creating my children’s local account on their computers, and having to send a copy of their ID to every platform under the sun, I’d easily chose the former.
I’d even agree to a simple protocol (HTTP X-Over-18 / X-Over-21 headers?) to that.
Which we have already seen with content ratings. Instead of using the rating to inform themselves on what content to allow their child, they basically relied on the retailers/theatres not selling access to people below the age.