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 not used to it. Mutton is much more popular in the Middle East and Asia.
Goat
The meat produced from an adult goat is called chevon and the meat from a goat slaughtered at an age younger than 12 months is named kid. However, mutton is also used interchangeably/commonly for goat meat.
Both goat and lamb meats are classified as red meats, however their nutritional composition is quite different. Lamb or mutton has more than double of the amount of calories, compared to goat meat. Lamb meat also contains almost ten times more fat and consequently more cholesterol. Naturally, lamb meat is higher in saturated fats, but also higher in monounsaturated and polyunsaturated fats as well. Goat meat on the other hand is higher in its protein concentration.