I first smoked at 13-14 and it was a mistake. I think that's a bit to young for most kids, but maturity varies... 16 is the age I would want my child to wait for before committing to doing drugs already.
If you start smoking (in our current system) at 13, you're pretty much bound to be doing harder drugs by 16 simply by exposer to street dealers - something a 15-16 year old shouldn't have access to because, from experience, teenagers are prone to making bad judgement calls. Hell, I am still prone to making poor decisions and I'm much past 13.