Have you ever forgot to brush your teeth? Have you ever forgot to turn off a light, pick up batteries, watch a show you wanted to, etc.? Just because you weren't carded one time doesn't mean that that place gives everybody that treatment, the person who was supposed to probably forgot that one time. Here's the thing, do you believe that one person deserves to lose their job and get massive fines because of a small mistake they made one time? If you report them to the police, that person is going to lose their job, get fined, and possibly go to jail. You said yourself that even you think that the drinking age is too high, so bearing that in mind, is it worth it to cause one person so much grief because of a mistake they made one time?
Sometimes, if you've been to the same place multiple times, if they remember you, they won't card you. The convenience store near my house is like that, I go in there all the time, they know that I'm over 21, so I don't get carded every single time. If someone who definitely looks under 21 goes in there, they're going to card them, there's too much at stake.