thirties
n. plural-only B1 Intermediate US //ˈθɝtiz// UK //θˈɜːtɪz// thir·ties
n. the years between 30 and 39 in a person's life or in a century.
n. the period of time between the ages of 30 and 39 inclusive, or the years from 30 to 39 within a specific century.
She decided to change her career in her early thirties.
Many people find that they become more confident in their thirties as they gain professional experience.
The global economy underwent significant shifts during the nineteen-thirties, a decade marked by both deep financial depression and the rapid development of industrial infrastructure.
Usage
Always used in the plural form; typically preceded by a possessive adjective or 'the'.
Pitfall
He is in his thirty'sHe is in his thirtiesDo not use an apostrophe to make a decade or age group plural.