A British psychological study published in Evolution and Human Behavior in 2010 concluded that men and women, in general, continued to follow traditional gender roles when searching for mates.
A study conducted by David Buss investigated sex differences in mate preferences in 37 cultures with 10,047 participants.
In all 37 cultures it was found that males preferred females younger than themselves and females preferred males older than themselves.
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 social theories for age differences in relationships as well as suggested reasons for 'alternative' age-hypogamous relationships.
I assume you are speaking of a romantic relationship, but try to assess your question in a 'bigger picture' framework.
In many instances children are caring for their aging parents, for instance.
Age disparity in sexual relationships is the difference in ages of individuals in sexual relationships.
Concepts of these relationships, including what defines an age disparity, have developed over time and vary among societies.
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.
did females show a preference for males significantly younger than male preferences for females" and that there was a "consistent cross-cultural preference by women for at least same-age or significantly older men".