What is mutton and lamb?

The meat is classified as mutton or lamb based on the age of the sheep

In United States meats are classified by USDA. The sheep meat is classified as follow:
Lamb: Less than one year old
Yearling: 1 to two years old
Mutton: Over two years old.

Even in the lamb there are special ratings for "Dakota Lamb" for leanness and "Spring lamb" (Born between March and October)

What they sell in most of the grocery stores is "Lamb" and NOT "Mutton". It is legal to sell "Mutton", but there is very limited demand, so you have to look at specialty stores.