Well where I live the legal age is 18 but I've thought that it could be bumped up a year to 19 like some of Canada's other provinces. But even with the age limit, most kids I know started drinking around 15 when they entered High School (some a year earlier). I've admittadely drank with my parents before I swore off alcohol (except when cooked with). When grad roles around in June, half the grade 12 population is already 18, because of it many of the after grads sell alcohol. They have to check for ID but it matters little, you don't have to buy a drink to actually drink it.
I hear kids, in grade 10, talking about drinking,basically cause they're bored. I've also heard them talk about how they dated hot "chicks" before they, "knew what to do with them." Which quickly was followed by a scoff of, "well the booze makes that easier." Things like that disgust me, I knew the one kid back in Junior High too. He used to wear clothes that fit and was a pretty cool guy to talk too, glad that didn't change. /sarcasm
The reason I say 19 is mainly so you can keep alcohol out of High Schools a little better though I don't see much changing. 21 at least makes it out of reach of more High Schoolers, but kids find ways. =/