The military age should be raised to 21 immediately. Our armed forces capitalizes on the ignorance and malleability of teenagers. It's a much milder version of the use of boy soldiers by African rebel leaders.
You're just talking out of your ass, right? That's an interesting point that I've been considering about marijuana for some time, but not one that I've ever heard about cigarettes. If you have any studies to show that rebelliousness is the main cause of smoking, post them, by all means. But I think that the majority of people smoke because people around them smoke.
If your father, uncle, or friend is a cigarette smoker, you're far likelier to develop the habit yourself.
We've seen from the model of alcohol that, when a drug is available to high-schoolers,
it's proliferated greatly among their younger peers. Keeping smoking at 18 means that high school seniors can and will buy cigarettes for their friends, at the age when they're most vulnerable to develop the habit. Removing smoking from the high-school setting entirely would do a lot of good to prevent addiction.
Personally, I think the drinking and smoking age both ought to be 19. But changing so many laws to this new age would be difficult.