There's a couple of ways that could go down.
replyOne way is that you log in under a guest account and the guest account requires you to indicate your age. After your session is over, guest account logs out.
Another way is that the library has two sets of computers, ones set for adults, ones set for minors. You need access card to use computer and the librarian will give you the age-appropriate access card.
Another way is computers are set for restrictive (child) account by default. If you need adult access you have to ask librarian to unlock it.
It's been a while but as a 12 year old I completely broke the library system, installed cracked Starcraft, and refroze DeepFreeze afterwards.
replyNot to counteract your point, just as an anecdote I like remembering.