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.
You got the framing question wrong. You should have been asking if age limits should be implemented at all, and then whether the current proposals will work (which they won’t), and then whether they cause side damage (which they do).
And then you must understand the key point: once you build these surveillance tools, they will be expanded. You say “only 18” but once the framework is in place, why not add in “credit check” or “gender” or “nationality”.
And actually, we already know how the checks are implemented: they involve identifying people specifically. There is actually no way to do “only 18” checks; it is a physical impossibility. You always have to gather more data.
And finally, the basics of individual liberty as well as safe computing involve you choosing what software you want to run on your computer, and that you have control of your machine. For this type of age checking to work, it must take control away from you, the end user. And companies like Windows and OS X love it, because that would destroy the FOSS world.