After enough of the older crowd die (which the act of smoking inherently accelerates, so it probably won't take too long, just a few decades) there would not be

After enough of the older crowd die (which the act of smoking inherently accelerates, so it probably won't take too long, just a few decades) there would not be enough smokers left for most places to profitably stock tobacco products. Dedicated tobacconists would probably collapse, larger stores in high-population areas may still stock tobacco products, but for the most part I would envisage smokers would be driven to specialist stores with online ordering and firm age verification. Additionally, it would be far easier to monitor and audit the limited number of places that do stock tobacco ("secret shopper" stings and similar) to make sure they aren't being lax in their age verification. Essentially, by making it impossible for youths simply pay someone a few bucks + the cost of the cigarettes to purchase them, the effort barrier could be made high enough as to prevent new smokers due to lack of easy availability.