Re: the Legal age of 18
For the same reason a 20 years old can't date a 16 years old, but a 60 years old can date a 18 years old.
Not all laws make sense, but it's much easier to make generalized laws when it comes to these things. Kids might not be mature enough to understand how much to smoke or the dangers of smoking or they might pass it to people who are even younger and so on. I think it could be moved down to 16, though 18 sounds official.
Personally I've been smoking since I was 14, in the middle east those rules aren't very enforced.