Biologically I think we are fitted to marry in our teens, at least in terms of reproduction and families and such. But the reason we evolved in this way is because average lifespans used to be much, much younger than they are now.
[Caveat: The rest of this will be USA-centric because it is where I live]
Our society is actively pushing back adulthood. Even in terms of the past 30 years or so the amount of people attending college has increased dramatically. Add to that the increasing attendance in professional and graduate schools, and we're looking at a dramatically increased path to adulthood. I think the mentality associated with continuing school well past age 22 means that people tend to avoid that "adult" mindset until they begin their careers. How much the actual mindset has to do with a successful marriage, I am not really sure. However I think the
experiences (lots of dating, lots of different social settings) and
education of college definitely contribute to a better understanding of relationships.
I don't have the specific statistic on this, but I read not too long ago that for every year marriage is put off starting at like age 20, divorce rates increasingly decline until topping out somewhere in the 30's. Basically, the chance of divorce with people who have college degrees tends to be much lower.
- Just found a couple stats: for women who marry between 20 and 24, there is a 36.6% chance of divorce. And for women who marry between 25 and 29, the percentage drops to 16.4%. It further drops to 8.5% for women who marry between age 30 and 34.
- 59 percent of marriages for women under the age of 18 end in divorce within 15 years. The divorce rate drops to 36 percent for those married at age 20 or older.