I don't think this is an age issue, but, far more of a maturity issue.
I have a teenager and I would contest to his smoking at this point because he isn't responsible enough to remember garbage day, or pet feeding, without reminders and it's just not time for him to experiment like that yet.

Other kids, same age, same interests, I can see where it would be OK.

It's not as though my kid's a tard or anything, but he's simply flighty by nature and still kind of a puppy with his energy and attention levels. Not long ago I picked him up from school (where a uniform is required) and he was wearing gym shorts instead of the usual pants. I asked him where the pants were; he did not know. The boy lost his pants: he doesn't need to smoke just yet, seriously. (in his defense he was being picked up from track practice...but STILL!)
I have a good friend / neighbor who's son is the same age and our kids are good friends. She allows her son to smoke; both kids get along great, it's not a part of their friendship, they have so many other things to do and discuss. It just so happens that this other boy is the eldest of 3 other children and has a higher level of responsibility, and more of a need to chill out after babysitting and chauffeuring. My kid is an only child and the situation is just very different, although quite similar.

I read a response above describing that her particular children simply do not have time to get interested in herb yet, and my kids in that situation, too. In our home, pot is regarded with relaxation rather than partying, and, being a teenager, he'd rather play soccer than sit and relax with Mom and Dad.