I wont let my kids smoke till they're 15-16. I don't believe any true damage happens to their brains at this point, and it helps them cope with the bullshit high school delivers, and teenage growth also delivers.
I personally started at 16, my life has just been rising up since. made me realize high shcool was total fucking bullshit. now I'm almost finished with high school doing home school high school and I will be hitting up college before fall this year.
each to his own. if your child wants to smoke, let them. it's not going to do any more harm than saying 'NO.' I know there are studies saying that smoking in adolescence fucks with brain development but I personally have never experienced this, nor have the 10-20 people I know that started at age 13!!!
I mean honestly, there are far worse things your kids could be doing, like eating mass amounts of junk food and playing video games all day.
edit: I smoke with my mom and my little bro. he's underage but it's helped him a lot and he's realizing the true significance of marijuana. if my mom didn't smoke with me I have no fucking idea what I would do.
well, I grow with my mom now but before she would share and I would also help contribute to the stash by working for other growers around here. good life