17. I've only seen that with heavy use and not in 100% of heavy users.
No psychoactive substance is completely benign. Neither is weed. However, adults in a free society should have the legal right to partake or leave it alone as their own lives and consciences dictate.
Prohibition never works. All it does is create conditions for gangs to arise and a black market to flourish. Whatever substance is prohibited always gets through and people who should not be labeled criminals for casual use are thrown into prison.