Then even if it is on steam, dude can't access it, makes the question meaningless.
While i don't know much how these verifications could be done in a safe way, i wouldn't want my ID info be uploaded anywhere, nothing is safe enough online.
Also, even if they require some kind of age verification, couldn't these things be tricked somehow?
I can see valve not wanting to deal with these kind of things, many companies rather not deal with them and rather ban things they don't want to deal with.
People find a way to access stuff if they want to, we've all bee there(most).
(We're kinda getting off topic too)