Your retired your horse from competition when he was 16. Now, four years later, your young niece is ready for her first show horse. Is it foolhardy to consider bringing your old campaigner out of equine retirement for her? Not at all.
Unlike people, horses retain fitness for years after their activity level declines. An older horse who was athletically fit in his younger years usually will have little trouble regaining his form when he's properly reconditioned. This means carefully increasing the length and intensity of his work and providing plenty of rest—two days a week—for his body to adjust to new demands. The one potentially limiting factor is soundness. If lameness was the reason for gambling a horse's retirement, increased activity may exacerbate the problem. In addition, previously undetected arthritis could become apparent as he's asked for increased effort. A horse's bones don't become brittle with age, so you needn't worry about creating new troubles. His tendons and ligaments do stiffen but are unlikely to cause problems unless he is regularly galloping at racing speed. Before putting a retiree back to work, schedule him for a full veterinary checkup. Then be mindful of lameness issues as he's carefully reconditioned and intervene quickly should trouble arise.