It's tough to get old but it beats the alternative!
Cushings disease is due to an overactive pituitary gland. The classic sign is a failure to shed and a long silky hair coat. Affected horses can be either over or underweight depending on whether it is complicated by insulin resistance. Diagnosis is sometimes made just on clinical signs but is confirmed by an elevated level of ACTH in the blood. Certain affected horses are only picked up byTRH response test. Cushings affects quality of life in many ways but the most serious side effect is laminitis. The drug pergolide (trade name Prascend) has made a huge difference in many older horses allowing them to continue useful lives well into their twenties. Compounded versions of this drug are in general less effective.
Arthritic conditions increase with age but be helped by NSAIDs especially firocoxib (Equiox or Previcox) or specific joint therapies.
Dental issues also increase with age. Regular dental care can help pick up small problems before they become big ones. Many older horses will do very well on senior type feeds despite missing some or even many teeth.