I think, once a child lights that cigarette, they are only responsible for the areas they are in . Meaning , just like all other smokers, they cant smoke inside, or in a working serving kitchen..etc
Though it does seem weird, that they cant BUY them, but can use them.
If you take alcohol for example, it is illegal for a person under the age of 21 to even posess it. Even if they are not drinking it, they can not have it.
If they are drunk, and under the age of 21, that too is illegal.
But cigarettes? Nah.. smoke away.. just dont buy them.