is true that a horse is more prone to Cushings(PPID) as it ages; older
horses have had more time for oxidative damage to the neurons, which,
over time, will result in PPID. However, they can still be baseline IR,
even when skinny. Being skinny can aid in control of IR, but it can
also be a sign of advanced IR.
There are a lot of factors that contribute to weight loss in an older horse (or the horse who was an easy-keeper becoming a hard-keeper). The obvious ones, such as teeth and parasitism, are usually fairly easy to identify. However, an older horse whose teeth look just fine may no longer have the muscle mass and muscle strength to chew as thoroughly as a younger horse, and so the hay isn't chewed enough to allow proper release of nutrients. Also, there is a gradual decrease in stomach acid production with age, which will reduce protein absorption. In elderly humans, there is reduced production of digestive enzymes, and reduced absorption of nutrients in the intestinal tract. It is possible that the same is also true for horses in the small intestinal portion of the tract, at any rate.
A short check-list of factors contributing to weight loss or hard-keeping would include:
Age-related changes in enzyme production, acid production, and ability to absorb nutrients
Teeth - either worn, or needing floating
Chewing ability - lack of strength
Underlying disease - kidney or liver issues, other disease states
Pain - laminitis, arthritis, other
Uncontrolled, advanced IR
Note that uncontrolled PPID will pack a triple whammy with reduced muscle mass and strength, and suppressed immune system resulting in a greater susceptibility to parasitism.
All of the above weight-loss "strategies" come with a price, however - not only are there fewer calories being absorbed, there are fewer essential nutrients being absorbed, and that is not a good thing.
Clear as mud? Sorry, I meant this to be a simple and short answer, but equine metabolism is anything but simple! There are lots of variables involved. Hope this helps a little.