I'm not suggesting a minimum age, I'm suggesting a minimum birthdate. So the minimum age will advance automatically - this year it will be 18, next year 19, etc... Put another way, people born on or before 2000 will continue to be able to buy nicotine, but people born after 2000 will never be able to buy it. Over time the percentage of the population which can buy nicotine will gradually dwindle to zero, and there will be no more excuses for stores to stock it.

AFAIK that approach hasn't been tried anywhere. I think I heard a while back it was discussed in Russia but never actually happened? I don't see any good argument for not doing this.