mid-thirties

The word is a modern English compound formed by combining the adjective 'mid,' a direct descendant of the Proto-Germanic midja- and ultimately the Proto-Indo-European médhyos (meaning "middle"), with the plural noun 'thirties,' which derives from 'thirty.' 'Thirty' itself comes from Old English þrītig, a compound of þrīe (three) and -tig (a group of ten). The logic of the compound is transparently literal, designating the middle portion of the fourth decade of life. This pattern (e.g., mid-twenties, mid-forties) is a productive and precise way in English to indicate a specific, non-exact age range within a broader decade.

Structure: mid (middle) + thirties (plural noun) - mid: From Old English midd, meaning "middle." It functions as a combining form to denote the central point or period of something. - thirties: From Old English þrītig, meaning "thirty," plus the plural suffix -s. It functions as a noun referring to the decade from 30 to 39 years of age or the numbers 30-39.