It depends. Every time I entered canada they asked for ID but sometimes they just barely glanced at it and asked the mandatory questions and we got through. Sometimes we had to wait a bit, i don't know what they were doing, don't know if they ran our info or what, but getting canada is usually really easy.

If you are underage to drink in the US, and you got to canada to drink, don't come across the border drunk, or smelling of alcohol, even if you're not driving, and act respectfully toward the customs agents and act like you are taking them seriously. i.e., don't put your feet on the dash, say yes ma'am, yes sir, etc. Sometimes you'll get an asshole.

One exchange at customs where my friend had his feet on the dash and we were still a little drunk, even thought the driver wasn't involved him telling us he would search our car to "teach us some respect" and even though we were'nt driving drunk "I think I need to get the state involved in this." even though his supervisor chewed him out in front of us for it.

Anyway, take them seriously to avoid trouble, and you shouldn't have a problem