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-fifties is a noun meaning "the time of life between 50 and 60". It carries one recorded sense.
the time of life between 50 and 60
Definitions, synonyms and semantic relations for mid-fifties come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.