actually, people aren't attracted to those in their age group. it was found that adolescent boys fantasize about slightly older women because older women (18-25) are more fertile. when i was 16, my first girlfriend was the married lady next door who was 23.
it was also found that people's first marriage are with someone who's close in age. but when men remarry, they tend to choose younger women (relative to themselves) with each subsequent marriage. older men, if they had the choice (in other words money) will marry a woman with as high reproductive value as he can get. in other words, he'll choose a younger woman. how often do you see a man cheat or leave his wife for a woman his age or older?