horses can go downhill fast. He looks quite skinny to me, so no reserves.
Animals appear to have the same age related problems as people; they can get winded just walking, stumble, loose their balance easier, not be able to chew foods, not be able to handle extremes of heat and cold, etc.
Which one each horse suffers first, is a mystery.
The first sign seemed to be with the feet, slower growth and more cracks, less resistance to bacteria and fungus.
With my horses, I noticed that they were breathing hard with just a good forward walk. Some I could ride only 15 minutes, some 30 before they started looking winded.
The next sign was eating hay slowly, leading to not eating hay at all, or very little.
The point that they can't eat hay anymore is close to the end. I will try feeding alternatives, like soaked hay cubes and lots of senior feed, but it is just a mater of time before they just seem so weak, and it costs so much to feed them, I know it is time when they seem rather listless. The other horses will often pick at them too. It is sad, but I don't let them suffer long.