Cats are now living longer than ever with the average age being between 11 and 15 years (may be shorter for pedigrees). It is no longer unusual to find cats in their 20’s and even some reaching their 30’s. In human terms a cat ages 24 human years in its first 2 years and then 4 years every year thereafter. Therefore a 15 year old cat would be 24 + 52 (13x4) = 76 years in human terms. Old age cats, suffer various illnesses. Old age can also bring along aching bones, arthritis, kidney and liver failure, thyroid problems and cancer. An elderly cat should be blood tested once a year as many medical issues can be managed in the older cat if they are found in the early stages. Unfortunately, for example in the case of kidney failure - the kidneys can be 80% damaged before the owner notices anything is wrong. His teeth will also need checking!
If you have an older cat, you must make allowances, as you would for a human. It may feel warm enough to you, but for an old thin cat he may well seek out the heat of your lap.