I go into bars and i'm underage (not by much though). I can see your point of view actually. Normally the underages are pretty obvious though, they never look the bar staff in the eye and always look downwards and sound generally feeble. My friends and I normally get served no problem, but there again, we don't act like idiots and don't get horribly wasted and puke everywhere. The bar staff can normally tell if you're going to be a pain/idiot.