Drinking age varies province to province. It's 18 in half of them, 19 in some, and 21 in at least one.
Laws of being caught with beer under age is $1500 fine for both you and the person who sold/provided it to you. But people generally start drinking around 15 or 16, in many cases. Of course, where I went to school, only the dumb people drank a lot young, most people stayed away until they were 18.
I should also mention that Canada is very tough on drinking and driving. My city in particular, if you're caught your car is impounded, license is taken away, huge fine, and almost always jail-time.
Marijuana also varies. The laws for possession are small, comparable to getting a parking ticket. But selling it and driving under its influence hold very stiff penalties. Where I live, again, only the dumb people smoke it, though. But I've heard that out west (in BC) it's much more common.