Its for your safety. They could care less about alcohol in you. The reasoning is this: 22 year old guys + alcohol + 16 year old girls= a very bad combination. Same goes for guys and whatnot. It keeps you out of a situation that could be dangerous for you if the wrong circumstances occur. Its just to protect minors and save RCI's butt from a lawsuit.
Whenever ANYONEs card is scanned in any of the seascan machines, it pulls up your complete profile. It shows your cabin, age, dob, picture etc.
If you get in without someone noticing you, nobody will bother you.
Also remember, if your under 18, your card has 3 immediate giveaways on it: 2 holes- Meaning Under 18, M1- Meaning Under 18 and a Shadow of teens dancing meaning your a "guest (15-17)"