People who first start drinking once they're are legally allowed to start doing so excessively. That's always the case, no matter if you are 16, 18 or 21. Being from Germany, where the drinking age is 16, I was stunned to see those 18 year old "kids" in the UK behave in a way that reminded me of myself when I was 16. I suspect this is similar for 21-year olds in the states.