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.
It seems obvious to me that, invasive as it is - an OS-level "are you 18 yes/no" check at installation would not satisfy the "protect the children" crowd at all, nevermind too that immediately when/if it goes into action - every single user would suddenly have their OS downgraded to the kiddy-level unless they declare their age.