I was active on another board about 10 years ago (give or take a year) ... On that board were young men and women (most of them being between 15 - 25, although I'll focus on the 18 - 25 now) and they talked a lot about their relationships (even though it was a board dedicated to a writer & not relationships, but ok...).
These people often wrote that they found a guy/girl and in case the relationship wasn't over within the year I could often read that the person got engaged...
I often congratulated them, but I could tell in advance that their marriages wouldn't last, especially in the case of the 18 - 20 yr old girls.
1. They are too young. 18 year old people may be legally adults, but if you look around you, you'll still notice a lot of childish behavior...
2. There are 2 kinds of love. The innitial love, which includes the butterflies & believing everything the other person does is perfect, etc., and a love that's more like intimate companionship. The first kind of love usually lasts 2 - 3 years at most. If you experience the first love it doesn't automatically mean it'll be replaced by the 2nd kind of love.
Since most people get married before the 1st kind of love has passed, they'll never know if there's a link with the other person or not (In many cases there isn't).
3. IF they've lived together (the capital F wasn't a typo btw.) before they got married, the time was so short they hardly got to know eachother. In many cases, they never even lived together at all before they got married.
So if you want a successful marriage :
1. Stay together until both partners are well in their 20's. I cannot give an exact age, because we're talking about individuals here and everyone's different
2. Stay together for at least 3 - 4 years before you ask the other person to marry you (if you still feel a strong connection towards that person by then)...
3. Spend at least half of those 3 - 4 years living together...
If someone asks a person to marry him/her then, the chances of the couple staying together become much bigger... (of course there's never a 100% guarantee)