It depends on the dish you are cooking. Young lamb (under 3-4 months) has a more delicate flavor and is leaner than older lamb (4 months to 1 year). It is excellent for roasts or cooked quickly as grilled and fried chops, cutlets and noisettes. The stronger flavored older lamb is more suitable for casseroles where the mild taste of young lamb would be lost.
When a lamb reaches one year of age, its meat is sold as mutton. This is less popular and so less widely available than young lamb, however, you should be able to obtain it easily from a good independent or specialist butcher.