Many of the smokers I’ve known started smoking at that age or younger. When I was at school there was a playground back market for cigarettes.
Banning cigarettes for younger people now won’t stop that. Just as banning cannabis for everyone doesn’t stop those who want to smoking it.
Many of the younger people in my family now however don’t want to smoke. There has been a significant shift in cultural and health attitudes against tobacco consumption, without a ban being required.