From the famous novel "Jane Eyre" where couples have large age gaps in their relationships.

May = middle of spring time.
December = middle of winter.

Here using seasons to refer to a person’s age in life.
For example. A younger girl married to a much older guy would be also called "May-December" relationship. In this case, the girl represent the spring time and using winter to describe her much older husband.

Also "rubbing the cradle" can be used to describe this kind of relationships. However, in this case, it has a bit of a insulting meaning to it. Referring to the older person "rubbing the cradle" of the younger person.

Cradle = Where babies sleep in.

(Genders can be reversed. In a vice versa way.)