It is cliche to note that men are often attracted to younger women, and women to older men. The typical evolutionary explanation(page 9) is that men generally prefer younger women, because the latter are more fertile on average. Women usually prefer older men, because the men can command more resources on average, including experience, among other factors.
In recent years however, the phenomenon of the mutual attraction of younger men and older women is gaining attention. Older women, roughly referring to women aged 35 or older, are popularly referred to as "cougars," while the men who share a mutual attraction are referred to as "cubs."
While a few recent demographic trends may provide some insight into the higher frequency of this relationship pattern, there are some purer evolutionary explanations as well.
When it comes to why older women may sometimes choose younger men, the former may find fewer mating opportunities with preferred older men(page 6, last 2 paragraphs), and hence settle for younger. But, why would younger men find older women attractive?
Perhaps the reason is the same as that above for women sometimes dating younger men. It could be due to a lack of younger mate availabity. However, there may be an additional dimension to the attractiveness of older women to men, at least when the women look younger than their age. Perhaps these women send signals of genetically-determined longer fertility lives, and hence represent an optimal mating strategy even in absence of a scarcity of younger or same-aged women.
Of course, that last point is mere speculation, as I haven't found any research support for it yet. If any of you find some, please let me know.
Quantitative Psychological Theory and Musings