can an under 21 driving with beer?!

As far as the law is concerned, as the owner of the vehicle the contents of the trunk will be considered yours, no questions asked. If you are with someone who is over 21, a case may be made for that alcohol belonging to that person and not you. However, the law in most states allows police to attach possession of vehicle contents to the owner or driver of a vehicle, but this is a discetionary move on the part of the officer performing the traffic stop. If you are pulled over, and you give the cops reason to search your car, you can be charged for being in possession of alcohol while under age. It is not a criminal offence, but you may be fined and have the alcohol confiscated. This, of course, will depend on the state you live in. In Alberta, where I live, the drinking age is 18, so we don't typically have this problem. Short answer is, don't give the cops a reason to search your car - so don't drive after you've been drinking, and obey the posted rules of the road, especially speed limits and stop signs/traffic lights.

If there is either someone who is 21 or older with them, or the car is registered to someone else (say your parents) and the alcohol is in the back. When I was underaged I was driving my dads car with liquor in the back, car was registered to my dad so I said it was his and it wasn't an issue. However, if its beer it will most likely be cold, and that excuse might not work.

Most of these answers are clearly from people in the USA who are quoting US (and Canadian) Law. But for that 95% of the world's population that doesn't happen to live in the USA, let me say that - it depends entirely on the country you happen to be in.

In some countries - Saudi Arabia, for example, being in possession of alcohol is a heinous crime, regardless of your age and situation. In England anyone can carry alcohol around, regardless of where it might be situated. And anyone over 18 can purchase, carry and consume alcohol.

im canadian but i think if your under the legal age your not allowed to have it in your possesion but throw it in the trunk and chances are you wont get pulled over, if you do then just act normal and they wont check your trunk lol

not unless there is someone who bought the beer or whatever in the car then they have already committed a crime.

drinking or not under aged is illegal in most states regardless of the situation

Not that I am aware of. Even if it were legal though, it would be very suspicious.