Lamb is the meat of a young sheep, whereas mutton is the meat of a mature sheep. There's some difference in flavour, but essentially it's the same animal. 中文没有这样的，都是‘羊肉’. In English, the trend is to remove the name of the animal from the meat that comes from it

In addition to Simon's answer, we have an expression: "mutton dressed up as lamb". This is used to colloquially to describe a middle-aged woman to tries make herself look younger by dressing like a much younger woman. It's not a flattering thing to say.