several riders at my barn keep their horses on purina's active senior feed. it keeps their horses in good weight and full of energy
Products for senior horses are very custom to a horses needs. Some senior horses need specific minerals and vitamins others do not. Contacting your vet (even if it is just a phone call) can help you.
Smartpack also has many options for joints!
You can use a gel pad under your saddle and creams to help with any muscle or joint pain the horse may be experiencing. You can also purchase a massage brush to use on the horse after competitions. Hope this helps!