When you talk about the thirties, you are referring to numbers between 30 and 39. For example, if you are in your thirties, you are aged between 30 and 39. If the temperature is in the thirties, the temperature is between 30 and 39 degrees.
双语例句
例： Mozart clearly enjoyed good health throughout his twenties and early thirties.
莫扎特显然在二十几岁和三十刚出头时身体很好。