When you talk about the nineties, you are referring to numbers between 90 and 99. For example, if you are in your nineties, you are aged between 90 and 99. If the temperature is in the nineties, the temperature is between 90 and 99 degrees.
双语例句
例： By this time she was in her nineties and needed help more and more frequently.
到这时她已经九十多岁了，越来越频繁地需要帮助。