I don't know the story, but just from what you've told me, this is my opinion: I'm fine with people marrying with big age differences. HOWEVER they still need to have been old enough to have a life and develop a sense of who they are beforehand, and I don't care how mature she is for 16....she's still just 16. Mature "for" an age doesn't mean mature. And not understanding that makes her not all that mature afterall, in my opinion.
The difference between 2 high school sweethearts staying together and getting married is that they are on the same level. They are starting life together, they will grow together, they are on an even playing field. (That said, I personally can't imagine having had ONE bf my whole life....it makes for a sweet fairy tale, and I know ONE AND ONLY ONE successful couple that did that, that are still together at my age....but they are still weirdly young, act young, head over heels in love but still seem to be......18). I married a man that is 17 years older than me, and is the best man I've ever met, let alone dated. HOWEVER at the time that we met, I had lived several different lives, had been homeless, had picked myself up and made a life for myself, overcame addictions, went to school and finished, traveled the world, was a missionary, a youth pastor for an innercity church (at the ripe old age of 22), worked with gangs and all kinds of other issues, was recognized by Bush when he was governor for the success we had in our youth center, and was quite settled, and then began a new career as an inspector for industrial construction. I met Jeremy at 24, and at 30 we were married. I was not a child who didn't know who I was, hadn't experienced the good and bad of life, and was still impressionable. I was young-ish, but no guy was going to "shape" me into whatever he wanted by then. I'd lived a lot more than many 24 year olds. I think this man wants a little trophy, someone he can train and shape into what he wants, and borders on pedophile. (The youngest thing he can legally get). She's.......just a kid. But what kid would want an old man without her own issues??? When I was 18 and partying in New Orleans, a "dirty old man" to keep far away from and throw drinks on was about 30-35. Anything older than 35 was just crazy (until I was 24 and fell for my awesome husband). But again, we MATCH, he doesn't boss me around or try to train me into anything (he'd better not---I know where he sleeps!)