A decade starting with the year xx30, most usually the decade from 1930 to 1939.(noun)
The decade of one's life from age 30 through age 39.(noun)
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Use "thirties" in a sentence
"In other words, the way social security has worked since the thirties is a disgrace."
"You succumbed to the wife's demands when you were 40 and so, if I may say so, have had a pretty good deal: being spared kids in your thirties is a greater fortune than that visited on most men who dread their arrival ..."
"He plans to focus on the years from 1972-1977 when Coetzee, in his thirties, is sharing a run-down cottage in the suburbs of Cape Town with his widowed father."