A lot of older animals (and people) just do get smaller and skinnier with age, but medically any weight change is significant, so it's time to bring your cat in to see us.
The two commonest medical reasons for weight loss in senior cats are kidney and thyroid disease.
Older cats have older kidneys that start to leak proteins into the urine. Their pee also becomes more dilute, so they have to drink more to keep up. They sometimes feel a bit nauseous, so their appetite is less and they start to lose weight. Specific changes to their diet, medication and supplements can make a big difference to how your cat feels and, bluntly, how long they live.
The thyroid glands control the basic level of metabolism, and in some cats they become hyperactive. Affected cats typically become restless, hungry, vocal and skinny. It's a bit like them having their foot on the throttle all the time: it wears them out. This disease is also treatable, most commonly with either medication or surgery.
So get to it: book an appointment now! And as an aside, an accurate (digital) set of bathroom scales is a good investment not just for you, but for your pets.