One ex of mine got married at 17, another at 18 (Appalachian milfs and I have a thing for each other). Neither decision was good, and people may think it's only a year, but the difference is drastic. At 18 you're legally an adult, but at 17 you aren't. At 17 you lack rights that your adult spouse has, like to file for divorce and to open bank accounts independently.

The minimum marriage age should be the age of majority and I don't care if your culture dislikes it. If you don't trust someone is old enough to sign a contract, why are you letting someone sign that contract? If they aren't old enough to purchase pornography, why are you letting them enter into a legal commitment with a sexual component? If they can't legally independently demand divorce why are you letting them marry at all?

We had to put a line between child and adult somewhere and 18 seems a pretty good place, but crossing that line grants an immense amount of freedom and independence.