thirty-something vs mid-thirties

thirty-something and mid-thirties both are nouns.

Word	Noun	Adjective	Verb	Adverb
thirty-something	Yes	No	No	No
mid-thirties	Yes	No	No	No

As nouns, thirty-something and mid-thirties are synonyms defined as:

Other synonyms of thirty-something include thirties.

thirty-something (noun)	mid-thirties (noun)
the time of life between 30 and 40	the time of life between 30 and 40