Yours is a very interesting question, and I look forward to hearing the legal answer too.
I do sincerely hope you take my comments in the spirit of support and hope they are intended. In other words, try not to be offended.
I'm wondering how much of these posts/boards you have read. These pages are FILLED with tragic tales of what happens when marriages fall apart. Many of the people who have given us a window into their lives here had all the advantages one could ask for, and still, they are here, trying to pick up the broken pieces of their lives. I will not say that either of you are too young. I will not say that you should not get married.
I will say that you should both be very, very careful. I have friends who are in the 30's thinking of marriage, and I say the same to them. I am a strong advocate of counseling, but if that is not something you are both comfortable with, then get a couple books on couple's communication. Then, ask each other and yourselves, Why do you/we want to get married?
Because you love each other and want to spend the rest of your lives together? You don't have to get married at 18 to accomplish that.
Because you want to take care of each other? Be there for each other? Again, you don't have to be married to do that either.
If either of you answers, Because you can't wait to be together, or Because everything will be so much better when I/you are out of whatever situation, or Because getting married will 'fix' something...
All of these are BAD reasons to get married. Ask just about anyone who posts here, and they'll probably tell you the same. Marriage doesn't make problems go away. Marriage BRINGS problems/difficulties/challenges to your life. It takes a very strong, open, communicative, respectful relationship to weather the everyday, in and out of life. Starting under less than ideal circumstances, or with less than realistic ideas about marriage is an almost certain recipe for disaster.
And we haven't even touched on the issue of children.
I do wish you all the best luck, but I also hope you will 'help' your luck by being careful. They say that lucky people have less turmoil in life. I say that cautious people have less turmoil in life.