More like people are biologically encouraged to be attracted to the best possible mate (that whole evolution thing). It just so happens that this is generally women in their early to mid 20s and men in their late 20s and early 30s.
While standards for beauty may change to some extent across cultures and time, the attraction based on age has been very constant. In addition to the biological health of the species, humans perception of the world tend to change as they age. People of more similar age are more likely to have compatible and relatable life experiences and views (try explaining what the world was like before 9/11 to a teenager. They simply don't understand the huge shift in culture that resulted because they weren't aware of the world at that time and only know the world as it is today)
Ageism and sexism are problems in some areas, but biological attraction when forming relationships is not one of those areas.
While standards for beauty may change to some extent across cultures and time, the attraction based on age has been very constant. In addition to the biological health of the species, humans perception of the world tend to change as they age. People of more similar age are more likely to have compatible and relatable life experiences and views (try explaining what the world was like before 9/11 to a teenager. They simply don't understand the huge shift in culture that resulted because they weren't aware of the world at that time and only know the world as it is today)
Ageism and sexism are problems in some areas, but biological attraction when forming relationships is not one of those areas.