Personally, I think getting married before 25 is nuts. My best friend just ended a year-and-a-half log engagement because she realized, a eighteen, that she was, well, eighteen. She also realized how immature her fiance could be, and she'd missed it for two years. However, if the marriage thing is essential to the story, then go for it! The thing is, it's fiction, right? I'm a complete skeptic on love, but I like a good love story in fiction. I even smiled like an idiot when Nathan and Haley on One Tree Hill (juniors in high school, mind you) got married after a very, very short courtship. So, to your question, if it works in the story, it works!