Lamb is a sheep that is typically under one 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 a year, ideally 3 years of age. It is an intense red color and contains a considerable amount of fat. Its flavor is very strong and many 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.