imo, 18 is way too young. i actually thought 23 was the age i wanted to get married, until i decided that was probably too young.
i'd rather have a chance to develop more as a person, grow, become wiser, that kind of stuff. you know, that whole freedom thing as well, i don't want to be tied down too early. i wouldn't mind being in a serious relationship at that age, but just not getting married.
and plus, if i married him at 18, it'll mean i would only know him for a yr. to me, that is a bit short of a time to decide whether or not they are the one for you.