A person's nineties are the period in which they are aged between 90 and 99

She was well into her nineties when she died. 她去世时已年过九旬。

the range of temperature between 90° and 99°

Today it's supposed to get very hot - maybe up into the nineties. 今天预计会非常热，温度可能将高达华氏九十多度。

the decade (= period of ten years) between 90 and 99 in any century, usually 1990–1999

Bill Clinton was the US president in/during the nineties. 比尔·克林顿在九十年代／在九十年代期间是美国总统。