be on the right/wrong side of 40, 50, etc.

be on the ˌright/ˌwrong side of ˈ40, ˈ50, etc.

(informal, often humorous) be younger/older than 40, 50, etc: ‘How old is she?’ ‘On the wrong side of forty, I’d say.’
See also: of, on, right, side, wrong