No way would I be getting married at 20, and there is no chance I would be happy if my daughter did it.
You say you're old for your age- fair enough, you've done more than lots of 20 year olds. But by 20, I'd been in a long term relationship for 3 years, had a daughter at 18, suffered domestic abuse, split up with her dad, started a career while a single mum, and was in the process of buying my home- a lifetime of experiences. I was in no way at all old enough to commit my life to someone. I would have told you I was, I would have been very wrong.
A few acquaintances did do it at that age (though only maybe 3), and they are all divorced- they changed too much in their mid- late 20s, divorced well before 30th birthdays.
As for the age gap being a problem, then it depends on your view point. 9 years when older isn't a lot. But for a nearly 30 year old with a university student, it seems too big. I'm 32 now, the thought of a 23 year old partner is very puzzling to me. Those ages would usually be on opposite ends of the spectrum, with the older ones approaching a settling down age, and the younger one trying to enjoy everything uni has to offer! That said, if you've got an old head and he's got a young one, then you could meet in the middle.
He does sound like he treats you nicely, but why the rush for marriage and the future? Wait until you know that you're truly rest to tie the whole rest of your life to this man.