學個片語：＂Act your age, not your shoe size＂

If someone thinks you're being childish, immature, too young for your age, they can tell you to act your age, not your shoe size. This saying, 'act your age not your shoe size', only works for British and American sizes.

如果有人覺得你幼稚或不成熟，心智年齡跟做事態度跟年齡不相符，就可以這麼形容你！扮演你的年齡，而非鞋子的碼數，這邊指美國或英國尺碼，歐洲尺碼不通用，如以歐洲尺碼44來看，等於英國尺碼的10號。如果你穿英國尺碼10號，＂Act your age, not your shoe size＂意思是說你長大了，該扮演現在應有的年齡，而不是像十歲小朋友一樣幼稚！

From BBC learning English