one's 90s; nineties
Refers to the age range 90–99, or the decade of the 1990s depending on context. Commonly used with possessive or attributive phrases.
My grandfather, even in his 90s, takes a walk every day.
It's rare for a woman in her 90s to travel alone.
Kanji spelling; less common but still used in formal or vertical writing.
Compound of 九十 (ninety) and 代 (generation, era, age range).