As an ex-smoker (but not reformed smoker) myself, I'd be happy to prevent anyone else from picking up the habit, but not happy to force someone else to give up against their will.
At Hogmanay this year, everyone born pre-2000 will be legally able to buy tobacco products, and everyone born in the 2000s will not be able to.
In my opinion, the sensible thing would be to keep it that way forever, have a fixed DoB of consent rather than a fixed age of consent.