Seventeen and talking of marriage? I think you guys both have many years of coming to know each other and know yourselves. For some people who are particularly precocious I suppose it can work, seems like a lot of people marry later just as it takes them longer to finalize their sense of core identity.
Enjoy the relationship but I can't say I'd recommend getting too ambitious yet. Don't be cynical either, just know that your both playing it by ear and that yes - in both positive and negative ways, anything can happen on any given day. Very difficult to call it, especially when you both I'm sure have as much growing up ahead of you in the next five or ten years time.