In various subcultures within the United States (and I imagine other places as well, mutatis mutandis for different educational and economic systems), many people basically end their educations with high school graduation, fully enter the adult world, and get married within a year or so. It is not unknown for couples, who have gone to school together, to get married right after they graduate from high school. I knew a couple who did this, two weeks after graduation, and they have a very happy marriage to this day, 40+ years later, three children, several grandchildren, the son is a Lutheran minister. So while it’s not something I’d generally recommend, it can and does work.
But 18 is not 12, it is not 14, and it is not 16.