To be honest, I had no clue the difference between mutton and lamb, I thought they were the same thing. Not many sheep around these parts. I've had one or the other two times that I can think of, once pulled, and once a chop, wasn't fond of either, but don't know the age of the animal that showed up on my plate.
You might be thinking that lamb and mutton are just different names for the same thing because they are both domestic sheep. While that seems to make sense, it isn't entirely correct. They have distinct differences, mainly in their age. As you probably know, the age of an animal can affect the flavor and how tender the meat is. And that, in turn, affects how you prepare that meat.
Lamb
Lamb is a sheep that is typically less than 1 year old. There is little fat on lamb, and the meat can vary in color from a tender pink to a pale red. Lamb less than 3 months of age is called spring lamb. Spring lamb is extremely tender but has a milder flavor than lamb. Most sheep meat sold in the United States comes from lambs simply because mutton doesn't have much of a following in the U.S.
Mutton
Mutton is meat from a sheep that is older than 1 year, ideally 3 years old. It is an intense red color and contains a considerable amount of fat. Its flavor is very strong, and you might have to acquire the taste before being able to enjoy a meal of mutton if you're an American.
Mutton is much more popular in the Middle East and Europe than in the United States. The gamey flavor of mutton does tend to appeal more to people who also enjoy other game meats such as deer, wild boar, and rabbit.
