I think the results of THC are a lot more subtle. For a start, it is not known why people have a reaction to it. Sure, the biological process is understood: there are THC receptors in the brain, and this is the mechanism that promulgates the high.
But not every creature has them, for example, mice don't (IIRC).
Considering there is a pandemic of depression across the world (I think it's 1/6 people suffer from depression at some point in their life), adding more uncertainty into th mix is a little ill-advised.
I agree that it is better controlled if legalised, though; after all, you can't regulate an illegal product on the black market. Tax it, too.
I would put an age limit of 25, though, considering the research findings about mental impacts. (Or whatever the scientists decide is the safest minimum age for it.)
Also there should be a maximum dose, too, and like radiation it would vary over the "exposure" time.
Certainly de-criminalising drug addicts, and treating them with legal drug prescriptions paid for by the society is a contraversial policy, too. But if you calculate the crime committed by drug-addicted heroine users, removing them from the crime statistics (giving them free heroine would mean they didn't have to steal their neighbours' video cameras to buy it), then it is justified on those merits, alone! Take into account the victims of crime and the criminalising of a significant percentile of society, and you have a compelling argument.