Top grades of lamb are prime and choice. Prime has the greatest marbling and is reserved for restaurants, high-end retailers and farmers' markets. Choice has less marbling, but is still high quality.
Lamb comes from animals that are less than 1 year old. "Baby lamb" is less than 10 weeks old and weighs less than 20 pounds. "Spring lamb" weighs 20 to 40 pounds. Tenderness is determined more by the lamb's age than by marbling. Young lamb is pale pink, tender and mild. As the lamb ages, the meat darkens, gaining character and flavor.
More than any other meat, lamb can vary due to breed diversity, diet and climate. Grain-fed American lambs are larger and carry more fat than lamb from Australia and New Zealand. The taste of grass-fed lamb is influenced by the type of forage. For instance, lamb from Brittany in France is prized for a subtly salty flavor that comes from grazing in local salt marshes.
Most lamb sold in supermarkets comes from animals that have been fed grain and raised in feedlots. But food experts believe that meat from grass-fed lamb has a superior taste and a particularly tender texture. Grass-fed lamb is available at butcher shops and specialty-food stores.