Not sure what you mean by "given the choice", isn't this redundant when you say "doesn't stop people from smoking cigarettes if they want to"? If they want to, age restriction isn't going to stop them.
You aren't pushing anything to the youth either, tobacco commercials are no longer legal, and there's shit tons of PSAs and programs in schools how bad smoking is for you. I doubt there's anybody above the age of eight by now that doesn't know the downsides of smoking.
I don't think ALL adolescents are as dumb as you make them sound out to be either, some of them will want to try it and get addicted perhaps, but there's also those who wouldn't even bother. Just an age restriction from purchasing cigarettes isn't going to influence their decisions one way or the other.