one's 40s; forties
Refers to the age decade from 40 to 49 years old. Used for people, and sometimes for eras or periods.
He quit his company in his forties.
What hobbies are popular among women in their 40s?
Kanji spelling; natural in formal or literary contexts.
Common numeric spelling; widely used in statistics, surveys, and casual writing.
Compound of 四十 (forty) and 代 (generation, era, age range).