They implemented this change in Jersey a couple of years ago. As far as I can see, the number of kids on the streets smoking hasn't gone down at all
It's incredibly easy for kids to still get hold of cigarettes. Go into a pub (hardly anywhere will question you if it's before 9pm) and just use the fag machine.
Nice idea, and I hope it works, but I don't think it'll change the number of young people smoking by much. It'll make the whole thing more rebellious, which might even be a counter-effect.