Age is just one of the factors that influence the horse's nutrient requirements. Young growing horses need high protein and energy for growth (16% protein and 75%TDN) and because they consume small amounts of feed. In their first year, foals should consume 1 lb. of grain per month of age. Free choice pasture and hay should be available. Yearlings and older horses will consume more feed, so the protein and energy content can be decreased (14% protein and 70% TDN). Mature horses need 10% protein and 65% TDN, which can usually be supplied with hay and pasture. Requirements for mature horses increase with work or reproduction. The increased requirement for mature horses is primarily energy. Calcium and phosphorous are critical mineral needs for horses and particularly young horses. The ratio should be 1.2 to 1 with calcium being higher. A trace mineral and vitamin package should be included in all rations. A quality forage will provide fiber and many of the vitamin and mineral needs of the horse. Most feed companies provide three to four grain rations that match the requirements and stages of development of the horse. If you feed these feeds at the recommended rates (1-2% of body weight), you will be close to your horse's requirements. Also, each horse is an individual, and adjustments in the amount of feed will be needed based on body weight.

The following are more articles on feeding horses.
Basic Horse Nutrition
Nutrition of Performance Horses
Nutrition of the Broodmare
Feeding Young and Growing Horses
Feeding Light Horses
Creep Feeding Foals
Nutritional Needs of Horses
Guidelines for Feeding Horses