Well, to put in a little Canadian perspective... as well as my experiences as an 18 year old...
In a small side note, Canada also had a Prohibition, just much less notably than America, and I believe that following Prohibition alcohol was put under government control, and to this day most provinces have all or alcohol above a certain proof sold only in government owned stores, eg. the LCBO in Ontario, and the SAQ in Quebec. This makes alcohol quite a bit more expensive here than both the States and Europe (damn your cheap booze!).
However, Canada's legal drinking age is 18 or 19 everywhere, with regulations varying province to province, some with alcohol consumption by minors allowed under supervision, and different regulations against public drinking, many provinces require alcoholic beverages to be concealed when in public. All the American students here at McGill are positively thrilled! As am I, now that I am now officially legal...
I definitely find that now that I am legal, my alcohol consumption is much more relaxed. I can't see any practical way for the legal drinking age to be reliably enforced anywhere I've lived, and teenagers will always manage to find alcohol or marijuana for that matter. However, alcohol is sometimes a hassle (maybe 10% of the time?) to get when you're underage and I think it definitely encourages binge drinking. I don't think I would lower the drinking age more in Canada, except I think it should be a blanket 18. I think first year students in college and university are particularly at risk for alcohol poisoning in provinces where the legal drinking age is 19. Though it's probably impractical, I could conceive a system which allowed an even lower legal drinking age that simply required a simple test of education about the adverse effects of alcohol resulting in a sort of alcohol consumption license. It would have to be simple enough to make counterfeiting ineffectual, but enough that teenagers would actually learn about alcohol in the process, and perhaps make better decisions. I think the legal age in America should be lowered... but I can't really pass judgement on the situation in America despite that I imagine it to be similar for alcohol.
Invictus, you say that you believe marijuana to be much more dangerous than alcohol, but I disagree. I think that may only be the case in the States with the whole War on Drugs thing. Here in Canada, marijuana is technically illegal, but it's not enforced at all. If you walk around brazenly drinking from a beer in public, you'll be fined. If you walk around brazenly smoking a joint, you might be asked to put it out. I think that a lot of the problems in America associated with pot is because it's considered much more illegal down there. I have American friends who I talk to regularly and it seems they always have a story about a friend who's life was ruined by pot, because they became apathetic and spent so much money and had criminal records and became alienated and so on. However, I don't believe ever hearing a similar story here in Canada, despite that I have much more exposure to the pot smoking community here. As a result of it's de facto legality here in Canada, marijuana is much, much, cheaper than in the States and people rarely find themselves spending beyond their means on pot. Also, I haven't heard of anyone who's had any legal troubles because of their pot smoking habits here. I think most of the factors that ruin lives in America are caused by the harsher punishments and the consequent drive in price.
As to apathy, I don't really think that marijuana necessarily causes apathy, though I will consider the possibility it may cause more apathy in people who are generally more apathetic, but of even that I'm skeptical. There is a vast pot smoking community here at McGill University and other surrounding universities, and most of them are ambitious and will successfully contribute to society and actualize their potential and their happiness. Also, I don't consider myself apathetic. ; )