Not to sound arrogant, but I can speak from experience here. My wife and I have been married for 14 years (15 in April, 13th to be exact...I always remember it). We started dating when I was 17, and married when I was 20 (she's 3 years older).
I went through the EXACTLY the same thing you are currently going through. From the day we announced our engagement many, many people said "You're too young." or "You should experience life before you settle down." I wanted to experience life with her. Only one person at my office offered any support. She actually encouraged me to get the child rearing years over with early, when we still had the energy (she was in her early 40's and had a two-year-old that wore her down).
Suffice it to say it's worked so far. There's been rough spots, but no more than any other married couple (even supposedly "mature" married couples) experience. The main difference is not so much in your feelings for your fiance/spouse, but in your behavior. If you behave in a loving way every day to the person it's not very likely you'll "fall out of love". Talk, listen, offer support, discuss (don't argue) your points of view...basically treat each other with respect. Realize that he's going to do/say dumb things, and you'll do the same. Have room for forgiveness. Mostly, have fun.
My only advice, to save yourself any misunderstanding in the long run, is spell out your feelings to each other about all the uncomfortable or boring stuff, like sex (all aspects, even the "non-traditional" ones), children (definitely separate topic from sex), money, goals, retirement, career plans, dreams, chores/responsibilities, which way to hang the toilet paper rolls (top or bottom), etc. Basically a pre-nup without the legal formality. As hard as it may be DO NOT JUDGE any responses each other gives. Leave your emotions at the door. As much as you (both of you) don't like something the other may say, do not let anything hurt your feelings. Be as dispassionate and reasonable as possible. If disagreements arise, talk them over. Do not argue. The point is not to debate, but to understand. If something arises that cannot be settled and is a deal-breaker when it comes to the marriage, better to find out now. Talk over compromises, but do not try to force a compromise if the other person feels strongly enough about the topic that they will not budge.
As much as I love my wife, most of the marriage has been very much like the experience you'll have if you follow the suggestion above.
On a side note, my brother and sister-in-law started dating when he was 18 and she was 17. They dated for about six years, and have been married for about 12 years now. However, two of my sisters married young and both were divorced within two years. Another brother married recently but they started dating about a year after my wife and I married so I don't think much will change. All of us started "young" and it's about 60% so far for the family.