I have no problem with the legalization of marijuana. There should definitely be an legal age limit. I'd say 21, but 18 is better than no age limit. As far as the dangers of pot, well, just like anything, if you overdo it, it can be harmful. I don't see it as being any more harmful than alcohol.
Bob, being that I have 2 teenage children (and a third that was a teenager until recently) I have attended quite a few meetings about marijuana. It is a danger to younger children. As a teenager, your brain is still developing, and it can affect the way your brain develops. That is why the legal age should be 18 at the youngest.
I have to admit I did smoke a decent amount when I was in my late teens and early 20s. I don't anymore. In the 21 +years since my first kid was born, I've smoked pot maybe 3 times and never around my children. I like the smoking pot, but I certainly can live just fine without it.