I think that there are, as with most things, both biological and social factors.
The biological aspect is that men like younger women because they are more fertile and attractive. Harsh but true.
However older women also feel the same about younger men. That is biological.
The social aspect is that men feel more entitled to express and fulfill their desire for younger women because our culture not only tells them it's acceptable but encourages them to do so.
Not the same for women. Women aren't taught to objectify men in the same way.
The current one sided setup is a legacy of patriarchy where men had the money and power and a woman's capital was in her attractiveness and fertility.