It's common for horses to continue competing and working into their late teens or early 20s before they are retired. Older horses can remain in good condition for longer with the appropriate care and nutrition.
Specially formulated commercial feeds are available for veteran horses, as well as natural herbal and mineral supplements, all of which help to prolong a healthy life.
It's also vital to ensure that older horses, prone to allergies, are fed only the best quality, clean, dust-free forage (hay). A mix of meadow grasses is advised to avoid digestive problems. Cereals are a no-no for veteran horses, because their digestive systems are not so adept at coping with high amounts of starch, and it could lead to problems like laminitis.
If you are in doubt about feed and supplements for a horse of advancing years, seek nutritional advice from your vet.