My husband works for a hotel and I know when they see a young person checking in alone or in pairs, especially around prom time, they keep an eye on that room. The reason? Many fun-loving young people check in, have lots of booze in their bags, which they insist on carrying to the room, and then party and trash the room.
This isn't to say any of you have done that, but it's probably a reason why the age policy, if that's what you can call it. That's not to say that a young person can't check in, but they do watch them. I agree with an earlier post, if you look business-like, your ID probably won't be checked.
It isn't fair, though, is it?