The Senior Horse

Keeping a horse comfortable into their senior years requires an attentive caretaker backed by a knowledgeable team. It is important to seek the input from your veterinarian and farrier to help maintain the health of an elderly equine and to spot conditions that will need special treatment sooner rather than later. Recognizing changes and not just dismissing them as old age setting in is a large part of the responsibility assumed when caring for the senior horse.
Pain Recognition
As a prey species horses are predisposed to hide pain so as not to attract the attention of predators. Horses can be very stoic and not show the signs of pain until it becomes severe. Often the first signs of pain are changes in behaviour or attitude as in the following examples 

A horse that normally greets you at the stall door is now hiding at the back of the stall
Rigid posture
Guarding part of their body or reluctance to be handled
Head lowering squinting eyes limp ears
Teethgrinding
Increased flight behaviour
Aggression
More subdued than usual
Decrease in responsiveness
Flaring of nostrils
Looking back at the flank
Restlessness
Stoic or pained expression dull eyes
Awkward tail carriage or aggressively swishing tail
Decreased performance
Reluctance to perform tasks which have already been mastered
Reluctance to move
Reluctance to drink cold water indicates oraldental pain
Recognizing pain is not always as obvious as the horse thrashing or rolling violently on the ground or the horse that is limping. The suddenly quiet withdrawn horse may be suffering from the pain of a stomach ulcer or another nonvisible ailment. It is important to resolve sources of pain early on. Chronic pain can cause depression and stress. It can also have a negative impact on appetite the immune system and tissue healing and can increase the risk for developing gastric ulceration and colitis inflammation of the colon.
Be on the lookout every time you groom for lumps bumps cuts heat or swelling and pay particular attention if your horse displays sensitivity to touch. Also take note of any decrease in bowel movements as an early warning sign for colic. Knowing the baseline vitals for your horse is important to help you assess health. 
Colic risk does increase in senior horses. Gut motility issues are more common in older horses which can lead to an increased risk of impaction colic. Other issues in the senior gastrointestinal tract that elevate the risk of colic include tumours and increased parasite load. Older horses tend to have a reduced