Differences in age preferences for mates can stem from evolutionary mating strategies and age preferences in sexual partners may vary cross culturally.
There are also alternative social theories for age differences in relationships as well as suggested reasons for 'alternative' age-hypogamous relationships.
The theory predicts that preferred mate choices have evolved to focus on reproductive potential and reproductive investment of members of the opposite sex.
This theory predicts both intrasexual selection and intersexual choice due to differences in parental investment; typically there is competition among members of the lower investing sex (generally males) over the parental investment of the higher investing sex (generally females) who will be more selective in their mate choice.
A study conducted by David Buss investigated sex differences in mate preferences in 37 cultures with 10,047 participants.