mid-fifties vs fifties

mid-fifties and fifties both are nouns.

Word	Noun	Adjective	Verb	Adverb
mid-fifties	Yes	No	No	No
fifties	Yes	No	No	No

As nouns, mid-fifties and fifties are synonyms defined as:

mid-fifties (noun)	fifties (noun)
the time of life between 50 and 60	the time of life between 50 and 60
	the decade from 1950 to 1959