As early as 6 months. Meats can be introduced anytime after 6 months. I generally recommend starting the meat/nonmeat proteins after all the cereals, vegetables, and fruits have been introduced. My own kids started meats much closer to 9 months.
8 months. Start at age 6 months with baby cereal mixed with water or formula. Then try the small 2-ounce jars of baby food veggies. If that goes well, then try the baby food fruits; and later the meats. If making home-made baby food, just use similar flavors as the ready-made jars, such as peas, sweet potato, apple sauce, banana, lamb, etc. Use each flavor for 2-3 days before adding a new flavor.
6 months. Recent findings show that babies should be offered meats as a part of their diet earlier than previously recommended. Meat is a wonderful source of zinc and iron, both of which babies can be deficient in. http://www.healthychildren.org/english/ages-stages/teen/nutrition/pages/zinc-good-for-growth.aspx.
6 months. This is a good time to introduce meat because your baby's iron stores will have been used up. Meat, especially red meat, is iron-rich. Start with strained or pureed meats and progress to finger foods by 7-9 months.