Age restrictions don't say it's 'okay' for somebody above a certain age to use something. Cigarettes will screw up your body at 18 just as much as they will at 17. But at 18 you're, legally at least, an adult, the point at which our unstated social contract starts to prioritize liberty over safety. And setting it at such an age also helps significantly reduce the peer pressure to smoke.

You’re right, and that’s the reason why some states have gone further banning cigarettes that have flavorings including menthol.