USA Today has an article on how long many young-adult couples stay together before marrying. The article asks rhetorically if the new relationship pattern is "dating for a decade."
In some cases, the protracted pre-marital phase seems to stem from partners' having to live in different cities, for educational or occupational reasons. Only when the two partners are settled in the same location do they finally say their "I do's."
Physical separation isn't the only reason for delayed marriage, however. Among the ideas proffered by experts in the article are couples' desire to make sure their relationships can handle strains over the long haul, individuals' keeping their options open if a more attractive potential partner comes along, and the realization that couples don't have to marry young if they don't plan to have children (or only have a small number of them).
(Thanks to Sothy Eng for bringing the article to my attention.)