thirty · number /ˈθɜrt̬.i/
thirty
A2 the number 30
Dictionary examples:
twenty, thirty, forty
She got married when she was thirty (years old).
Learner example:
It's thirty years old. (Key English Test; A2; Italian)
(be) in your thirties/30s
B2 to be aged between 30 and 39
Dictionary example:
Most of my friends are in their thirties.
Learner example:
He was in his thirties and really handsome. (First Certificate in English; B2; Spanish)
the thirties/'30s
B2 the years 1930 through 1939
Dictionary example:
They moved to Paris in the thirties.
Learner example:
In the beginning of the thirties in [the] USA the film industry [grew]. (First Certificate in English; B2; Spanish)