Your rational is wrong. The reason children aren't allowed to buy cigarettes is they are too young to make an intelligent choice of whether or not to smoke. Punishing them for violating laws which exist only to protect children from their own lack of judgement does not make much sense to me.
I disagree. Those under 21 can be fined for attempting to purchase, possesing, and/or consuming alcohol. Why not cigarettes then?
And there is NO law against those under 18 purchasing cigarettes. If a 15 year old buys a pack of smokes they have broken no law. The person who sold them the smokes is the only one who has borken the law.