At any age younger than eighteen, you are still considered to be a minor. Most people's body's are still growing even after they turn eighteen and become adults. At this age, smoking can, and most likely will, stunt your growth.
Chat with our AI personalities
Perhaps that is the age at which self degradation is considered to be allowable.
You can smoke at any age, but you cannot buy cigarettes until you're 18. You can only smoke in a public place when you're 18. But at home any age, at least if not caught.
I assume its because that's the legal age of concent, and as cigarettes can cause health problems, you need to be 18 so you can take responsibility for your own health. Just a guess. :L