You might think lamb and mutton are simply different names for the same thing since they both come from domestic sheep. While that assumption might seem logical, it's not quite correct. They have distinct differences, primarily in their age. A lamb is a sheep that is generally less than 1 year old. It has minimal fat, and the meat varies in color from a tender pink to a pale red. Mutton comes from a sheep that is older than 1 year, ideally around 3 years. It is a deep red color and has a considerable amount of fat.

Source: The Spruce Eats