We’re not talking about what they want. An outright ban is an option, here. The goal is to make them smoke less. To make them less addicted. Lowering how much nicotine they get, without changing their habits, would probably help immensely.
Though half at once is the wrong curve. You’d want to drop by 10% a year. Enough to grumble about… not to double how many you smoke in a day.
“Upping the age every year” is an asshole move of the highest order: inequality. You’d tell some people, this is legal, but never for you. That is fundamentally the opposite of ‘you must be 18’ and it cannot be tolerated, even if the motivation is positive.