Face, frame and coat
The classic portrait is real: white hairs salt the face, especially around the eyes and muzzle, and the hollows above the eyes deepen as fat pads shrink. Along the back, the withers stand prouder and the spine shows more as the muscles that once upholstered his topline waste — the sway-backed look that no amount of feeding fully reverses, because it is muscle loss, not fat loss. His coat may grow thicker, shed later and lie flatter than it did, and his skin loses some of its old elasticity. His cheeks hollow as well, as the muscles of the jaw shrink along with the rest of him. Judge all of it against last season, not last decade.
Teeth, weight and appetite
Aging mouths wear down, loosen and sometimes lose teeth entirely, so an old horse chews more slowly, may dribble feed or drop quids of hay, and extracts less from every mouthful. Weight follows teeth: many seniors become harder keepers each winter, ribs showing despite unchanged rations, because the feed is passing through less completely chewed. Watch his droppings for long, undigested fibre, watch him eat rather than just watching the bucket empty, and treat drinking changes — much more or much less — as information worth reporting.
Movement, rest and recovery
Stiffness that warms out after ten minutes of walking is an old horse's familiar signature, along with shorter strides on hard ground, reluctance on slopes he once took easily, and more time lying down — plus a little more care needed getting back up. He may sleep more deeply and startle more when woken. After exertion, everything takes longer: breathing settles slower, enthusiasm returns later. None of this demands alarm, but all of it asks for management — softer ground, gentler work, deeper bedding, longer warm-ups.
Build a baseline while he still looks well
The changes of age are only recognisable against a younger version of the same horse, so make that version easy to recall. Photograph him from the side, front and rear on the same patch of yard each season; run a weight tape round him monthly; keep one line of notes on how he eats, moves and rests. None of this takes long, and it converts the stable-yard argument — 'he's always looked like that' versus 'he's changed since spring' — into evidence you can act on and show your vet.
What is not 'just old age'
Age explains slow change, never sudden change. Weight falling off in weeks, a horse off his feed, a winter coat still hanging on when every other horse on the yard has shed, excessive thirst, new lumps, a dull or withdrawn manner, or difficulty rising that appears quickly — each of those has treatable causes often enough that a vet should see him rather than the change being filed under his birthday. The discipline that catches the difference is a written trail: a dated line each month on weight, teeth, coat and movement in his KinsLe Pet Passport builds the before-and-after picture that tells you, and your vet, what is aging and what is not.