When you talk about the fifties, you are referring to numbers between 50 and 59. For example, if you are in your fifties, you are aged between 50 and 59. If the temperature is in the fifties, the temperature is between 50 and 59 degrees.
双语例句
例： I probably look as if I'm in my fifties rather than my seventies.
我可能看起来像五十多岁，而不像七十多岁。