"Just getting old" is usually pain.

Ageing itself isn't painful — the conditions that come with it are, and many are manageable. The tell is behaviour rather than lameness, because animals rarely limp until it's advanced. Look for things they've stopped doing, stiffness after rest that eases with movement, jumping in stages rather than one movement, reduced tolerance of handling, and in cats a scruffy coat from grooming less. "Just getting old" is the single most common way treatable pain goes untreated for years.

Cats almost never limp. Joint disease is very common in older cats and very rarely diagnosed, precisely because the signs are quiet: no longer jumping to a favourite height, jumping in stages, going outside a high-sided litter tray, an unkempt coat toward the back end, hiding more, and being less tolerant of being handled. Every one of those gets attributed to age or personality.

Because it's the single most useful thing you can bring, and animals behave completely differently in a consulting room — a stiff animal often moves normally on adrenaline, and a frightened cat may simply freeze. Short clips of them rising from rest, walking directly away and back, and taking stairs show a vet what an examination may not. Film in daylight, at their level, ten or fifteen seconds each.

Weight, and it costs nothing. Excess weight is the single most modifiable factor in joint disease, increasing load on every joint — no other owner-controlled intervention matches it. After that, home adjustments work the same day: cover slippery routes, add a step to whatever they jump to, switch a sore cat to a low-entry tray. Any weight reduction plan needs setting by a vet, and for cats it must be gradual because losing weight too quickly can cause a serious liver condition.

Not from your own medicine cabinet, and this matters because a sore-looking animal is exactly when people reach for one. Paracetamol, called acetaminophen in some countries, is severely toxic to cats and a single tablet can be fatal, and anti-inflammatories made for people are dangerous to both cats and dogs. Supplements are also not pain management — the market is unregulated and the evidence variable, so they're a question for the vet rather than a substitute for an examination.