Marijuana is pretty much harmless, worst it can do is give you cancer really, and that still is rare.
At the age of 16 you are allowed to get your license and drive legally, putting your life, and others lives at great risk.
At the age of 18 you are allowed to join the military, but you aren't allowed to legally drink a single beer.
Marijuana use is only really something that will affect the people using it, and the people who aren't using it will be fine. A lot of people you talk to daily are problem stoned during the time you are speaking to them, and you don't realize it, why? Because smoking weed barely does anything to you.
This also comes to my want of them to lower the smoking and drinking ages. My 2 statements above back that up I'd say.
Marijuana should be legalized, all it does is make the user feel good, and it's socially acceptable amongst many groups of people, I know a lot of people who smoke weed together all the time, without it we would have fun still, yes. But the weed is simply just a bonus to hanging out with people. It is harmless, and it is a fun activity. It makes us feel good, so why keep it illegal?