An older man dating a younger woman is often referred to as an "age-gap" relationship. In some contexts, especially if the age difference is significant, the term "May-December" relationship is used, symbolizing a spring-autumn life-stage difference.
The term "sugar daddy" is sometimes used, but it typically implies a financial arrangement or transactional relationship, which is not the case in all age-gap relationships.
There is another "manther" describing a male version of women's "cougar" - older men who pursue partners significantly younger than them.