Schooling plays a bigger and bigger role in mate selection. As the most educated people spend longer years in school, and with a more homogeneous pool of people, the most educated couples are likely to get more similar to one another in education, age, and status level.
Something I had not thought about until reading Who Marries Whom, which I will be blogging on for the next few posts, is that the least educated couples will be less similar to one another in education, background, age, and status level, than the most educated couples will. There is more variety at the bottom than at the top of the educational ladder, and more time and social locations to choose in. The biological clock is not ticking so loudly for high school women as it is for professional school women, so they are less likely to marry the person they were dating when they finished school.
Educational homogamy is not uniform throughout the social ladder.