The truth is no one has been analysing cat lifespans. These are estimates or to be more generous "guesstimates" based on a reasonably assessment.
Another point to make is that a full-time indoor cat might not, on average, live longer than an indoor/outdoor cat.
ASPCA says that the average lifespan of an indoor cat is 13-17 years with some living to 20 plus but that is not an average is it? That is a very generous spread based on informed guesswork.
|My late lady cat aged about 16 years of age at the time. I had just wiped her coat|
which is why it looks a bit wet. She lived four more years.
Petco says that an indoor cat has an average lifespan of 12-20 years (even more of a spread and not an average in my view). They also say that outdoor cats (cats kept outdoors full-time I presume) live for 1-5 years. This is highly simplified. No one keeps a domestic cat full-time outdoors - well, very few people do. I suppose they are referring to feral cats but don't say that. This is neither scientific nor accurate information.
Wikipedia confidently state 12-15 years as an average "life expectancy of a cat". They must mean the domestic cat as opposed to the feral cat.
Wikipedia quote a study as assessing the feral cat lifespan at 4.7 years. The links to the study are broken so I could not read it.
Indoor cats need more play to make them active. People in general don't play enough with their cats. An indoor cat arguably is more likely to become obese and obesity is the cause of many serious illnesses which can be life threatening. Outdoor cats within a secure enclosure will get exercise safely. This is rare however. People aren't bothered to build decent enclosures to be honest.
We know that outdoor cats, meaning stray cats and feral cats, live shorter lives than well cared for domestic cats who live mostly inside the family home. We can probably rely on 14 years as being a reasonable guess at the domestic cat lifespan. I'd favour a bit longer say 15 years. Like people cats are living longer but also like people dementia is more commonplace. Are cats living too long sometimes?
Feral cats will be lucky to live beyond five years on average but many may live to a not dissimilar age as domestic cats if cared for by a feral colony carer i.e. a person who feeds and TNRs the cats as part of a program. Many feral cat carers provide winter housing as well as feeding. There are many factors so averages don't really help. It is about individual cats.