From Scientific American:
Men marry younger women and women prefer to marry older men, in general. But is it culture, genetics or the environment that drives such a choice—and is there an optimal age difference? New research shows that, at least for the Sami people of preindustrial Finland, men should marry a woman almost 15 years their junior to maximize their chances of having the most offspring that survive.
“We studied how parental age difference at marriage affected [families’] reproductive success among Sami people who married only once in their lifetime[s],” says ecologist Samuli Helle of the University of Turku in Finland. “We found that marrying women 14.6 years younger maximized men’s lifetime reproductive success—in other words, the number of offspring surviving to age 18.”
More here.