Why is it always 18 for women and not younger? I know state laws vary but The Church allows for younger than 18.
I'd be thrilled if my daughter were married at 16. I think though, to be married at that age requires a lot of resistance to the values and influence of the world upon teenagers. I'd definitely want a daughter marrying young to have an older and wiser husband who was established enough to have the wisdom and knowledge to guide the marriage.
Unfortunately, compulsory attendance laws for schooling are getting pushed to 18. I think this is a factor, as well. I do think a young lady should have enough education to be able to educate both boys and girls through high school, that way public school is never a temptation.