1. the numbers from 30 to 39, especially when referring to temperature or age
Usage: plural form
2. the decade from 1930 to 1939
Usage: plural form; often capitalized as ‘the Thirties’

Examples

• The temperature will be in the thirties today.
• She’s in her thirties and thinking about starting a family.
• My grandfather was born in the thirties.
• The Great Depression occurred during the thirties.
• It’s supposed to stay in the low thirties all week.
• He’s in his early thirties but looks much younger.
• The thirties were a difficult time for many families.