I don't think it is the government's role to protect people from themselves. So, although I think smoking weed is downright stupid and leads to nothing productive, if you choose to do it, go for it.
But, I think the government has a role to play in two areas. The first is to regulate the quality of what it is that is available and to help inform people to make intelligent choices. That is no different than any food or drug product on the market now.
The second is to protect others. While it is okay with me if you choose to smoke, the second you do something to put others at risk, I draw the line. So, smoke all you want, but if you choose to get in a car and drive while you are impaired, we should throw the book at you.
Likewise, I favor dropping the drinking age to 18 but at the same time increasing DUI penalties to make them very severe. I lived in the UK for awhile where this is the case, and I can tell you that young people didn't even think about driving if they had been drinking... at least the ones I was around.
We should regulate the behavior that results from use of these things, but not the use of them per se.