Lamb and mutton both are domestic sheep, but have distinct differences, mainly in age. As you know, the age of an animal can affect how tender their meat is and their flavor.
Lamb is a sheep that is typically under 1 year of age. There is little fat on lamb and the meat can vary in color from a tender pink to a pale red. Lamb under 3 months of age is called spring lamb.
Most sheep meat sold in the U.S. comes from lambs simply because mutton isn't as popular.
Mutton is meat from a sheep that is older than 1year, ideally 3 years of age. It is an intense red color and contains a considerable amount of fat. Its flavor is very strong and may have to "acquire" the taste before enjoying mutton.
Mutton is much more popular in the Middle East and in 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, rabbit, and other more exotic animals.
Common Cuts of Lamb
Because Americans prefer the more delicate taste of lamb, it is more expensive than mutton but also much easier to find in regular markets. Common cuts in the U.S. are shoulder roast, rack, loin chops, and leg of lamb.
It's also important to note that domestic lamb tends to be grain fed, contain more fat and have a more delicate flavor. Imported (typically Australian or New Zealand) lamb tends to be grass fed, be leaner, and have a stronger flavor.
According to the American Lamb Association, the three most common cooking methods for lamb are grilling, braising, and roasting. Grilling (or barbecuing) over hot coals is great for burgers and lamb chops. They recommend dry brining and salting the meat about 40 minutes before cooking to help break down the proteins.
Braising is where meat is first browned in fat and then cooked slowly in a covered pan with a small amount of liquid. This can be done on the stovetop or in the oven and this method is great for tougher cuts such as the shoulder.
The dry heat of oven roasting is best for more tender pieces like the rack or the leg. The meat is cooked uncovered and produces a brown outside and moist interior.
Because mutton is tougher, a slow cooking method like stew helps to tenderize the meat and bring out the flavor.