one's 30s; thirties
Refers to the age range 30–39, or to people in that age group.
He went abroad for the first time in his thirties.
It's a brand popular with women in their thirties.
Standard modern spelling using Arabic numerals.
Kanji spelling; common in formal or vertical writing.
Compound of 三十 (さんじゅう, 'thirty') and 代 (だい, 'era' or 'age range').