There may be other preference-related explanations. Perhaps men are more likely than women to opt out of heterosexual relationships (e.g. by being gay or happily single).
Then there are demographic explanations. Our species is not perfectly gender-balanced; there are (slightly) more women than men.
Women are looking for a mate that will be able to give her offspring a good chance of making it into the next generation, thus what she is looking for is someone with abundant resources who will secure her home and future. This is also why old but rich men are attractive to women. Men, on the other hand, are basically looking for someone young that will be able to bear a child, and this ability degrades with age. Which is why men of all ages like young women.
The reason it doesn't work the other way round could be that for men, the quality of the genes of their mates don't matter so much: just get as many children out there as possible, some are bound to be OK (it is not as high an investment as it is for women).
Still puzzling why women can't bear children up to a higher age - forgot why that was. Apparently it is quite unique to humans.