Here's an interesting set of statistics. It seems to state that if you marry in the wildly changing personality years of age 20-24 that you are more likely to divorce than any other age group.
Let me see if I can get this to line up properly.
Female Male
<20 yrs old 27.6% 11.7%
20-24 yrs 36.6% 38.8%
25-29 yrs 16.4% 22.3%
30-34 yrs 8.5% 11.6%
35-39 yrs 5.1% 6.5%
So it's really best to wait until you are 30 if you want it to last.
This data is from the CDC. The other books I've read that talk about this information indicate that it's usually that the 20-24 age group is going through all sorts of personality changes, goal changes, etc. and someone they hook up with is not necessarily going to be the right person for them long term.