Senior horses are older horses, typically over the age of 15 or so. As horses age, their nutritional and healthcare needs can change, just like in humans. Generally, senior horses experience a decline in physical activity, leading to weight gain, as well as changes in their metabolism and digestive system. They may also be more susceptible to health problems such as joint issues, dental problems, and decreased immunity.
Feeding senior horses is much the same as feeding other horses: 24/7 access to hay is essential as providing them with a consistent source of good-quality forage will decrease the chances of problems to the GI tract and the horse's metabolism.
However, you may need to add in a probiotic and feed a higher quality hay for your older horses. In some cases, it may also be a good idea to switch to a senior-specific feed that is lower in sugar and higher in fiber.