Wouldn't this mean we don't know what age these 10% are in?
there are laws against underage drinking and buying alcohol. some kids still get access to alcohol. the law is mostly successful, with an acceptable amount of failure rate.
same concept.
Then your 10% becomes problematic because you are either restricting or granting access based on invalid information. So in your world here we then need ways for people who were incorrectly gated to reach out and be corrected somehow.
Some kids skipping the block sometimes isn't catastrophic, especially because there won't be all this peer pressure to join adult social media when 90% or more are blocked.
I'm not sure how you confidently can say that every company is going to allow the people we can't verify have access?
If you're talking about other unknowns, you need to explain why they're unknowns.