one's 50s; fifties
Refers to the age range from 50 to 59, or to people in that age group.
My father quit his job in his fifties.
What hobbies are popular among women in their fifties?
Kanji spelling; also common, especially in formal or literary contexts.
Compound of 五十 (ごじゅう, 'fifty') and 代 (だい, 'generation' or 'age range').