Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
mid-thirties is a noun meaning "the time of life between 30 and 40". It carries one recorded sense.
the time of life between 30 and 40
Definitions, synonyms and semantic relations for mid-thirties come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.