This is making a long story short but... I had my first serious girlfriend when I was 18 and she was 16. It was my first year in college and she was a Junior in high school. After a year of dating, I was absolutely certain that she was the one that I would spend my life with. However, I thought it would be smart to wait until atleast I had graduated from college (she decided to come to the same school, so distance was not an issue).
I am so glad that I did, because over the next three years it became apparent that marriage was out of the question any time soon. Basically, as you grow up you realize things that you could not have understood before. People tell you things, but you don't think they apply to you. For example, issue #1 is money... I think about the future, she thought about the present. I think I am pretty smart with money decisions (I bought a house, don't like to use credit cards, shop for bargains). She, on the other hand, spends her money as fast as she gets it (eats out all the time, doesn't realize how small purchases add up, etc.), which really bothered me. She would then need some money for gas or something like that, and I refused, which would cause arguments.
My advice:
Are your finances in order? Are you both on the same page in this area, or do you know? I know that when I was 19, living with my parents, I didn't have the capacity to even recognize a situation in this area (though I thought I did).
Secondly, what are your plans for college? If you are both going to the same school, why not just take classes together, live in the dorms close to each other, and basically spend all your time with each other. It is going to be the best time of your life and you can still share it together.
Over the next 4 years you are both going to change very much. Some people change together, others change apart. You are right, getting married young can work for many people. However, it doesn't work for many others. What is common between those who stayed married and those who did not... they both thought they would stay married and be happy, just like you. Sure, you know that she is the one, everyone who gets married does. But many people later find out that they were wrong.
Why not wait until this particularly volatile time in your life is over?
All that being said... everyone is different, and every situation is different. I don't know you at all, so I can't say anything in your case. However, there are people that know you. Talk to your friends, family, her friends and family. Get their honest opinions. Others can see things that you can not.
Jeez, that was long anyway.
