For all the people who say "yeah, but you can do [blank] at 18!", so-fucking-what? Why does turning 18 suddenly make you qualified to do everything or give you an adult perspective on the world? There are plenty of non-18 age requirements in the US. You have to be 35 to run for president, 30 to be a senator, and 25 to be a representative. You can't even get your full driver's license until you're 21.
"But you can fight and die for your country at 18!" you might say. Yeah, and that's kind of shitty. Maybe we should address the age limit for enlistment the same way California did for smoking. The fact that enlistment officers try and get you to sign your life away during one of the most confusing and transitional periods of your life seems a little too predatory, especially when they focus on kids who live in poor areas or have bad grades. (This point is irrelevant though, since people in the military are exempt from this age restriction)
"But you can buy a gun at 18 in most states!" Not without obtaining a gun license and going through a background check. Even then, buying a gun and using it for its legal purpose doesn't hurt anyone. Using cigarettes for their legal purpose certainly does.
"It's not like kids are going to stop smoking, so what's the point?" No law has ever stopped everyone from breaking it. That's not how this works. Besides, 18 year olds can still legally smoke, they just have to get someone 21 or older to buy the cigarettes for them.
5 of the 6 leading causes of death in the US can be contributed to tobacco in some way. Decades of data on the population's consumption and health show patterns that explain how this happened. People who begin smoking early, even at 18, have a higher risk of getting addicted than people in their 20s or 30s. This suggests that the average 18 year old is still at a disadvantage when confronted with tobacco products and can be taken advantage of by its addicting qualities. This age limit won't stop people from smoking, but it will discourage those who are still at a disadvantage if it isn't immediately available.