This is because the lives of outdoor cats vary tremendously. The life of an outdoor cat amongst a well cared for colony of cats in a relatively warm and stable environment, in an area where there has been extensive vaccinations of cats, might live as long as any full-time indoor, pedigree cat or longer. . Many feral cats live quite long lives. At the other end of the spectrum a feral cat living in a hostile urban environment with freezing winters may not live beyond two years of age. It all depends on so many variables. However, most “experts” rate feral cat lives at 2-5 years.
All we can say is that an outdoor cat is likely to live a shorter life than a full-time indoor cat on average. If the average age of a domestic cat is about 14 years then at a guesstimate we might say that the outdoor cat life expectancy on average might be about half that but it is rather pointless creating an average when the spread of lifespans of outdoor cats is so wide.
There are hazards which can shorten the life of both indoor and outdoor cats. It is these hazards which result in the outdoor cat having a shorter lifespan. Outdoor cat dangers include: predators, diseases and parasitic infections, fencing and boundaries, domestic refuse, other cats, outbuildings, pesticides and garden sprays, roads and cars, theft, traps, trees and water. Indoor dangers that are hazardous to cat health include: plants (safe ones), foods, kitchens and the equipment in them, bathrooms (the dangers of water), electric power points and appliances, sewing boxes, string, cat litter dust, carpets (the chemicals in them), furniture (the chemicals in them including fire retardants), DIY products, human medicines, open windows and balconies.
You could argue that there is an equally long list of hazards for cats living indoors as there is awaiting the outdoor cat. Some of the indoor hazards are hidden. People don’t know about them such as the chemicals impregnated into new carpets. There is also the issue of stress and boredom which is more likely to occur indoors which could arguably lead to shorter lives due to the stress itself or obesity which carries with it diseases such as diabetes type II.
There are also many cats who live outdoors a large percentage of their time but also have access to an indoor environment. There is a range of cat lifestyles which also muddies trying to work out outdoor cat life expectancy.
As people are researching information about outdoor cat life expectancy, we should ask ourselves why people are looking for that information. The likely reason is that they are working out whether they should keep their cat indoors permanently or let them out and a major factor in the decision making is the lifespan of an outdoor cat. If I am right in this assessment then the better research might be, “how can I keep my cat healthy while allowing her to go outside sometimes in order to provide her with the natural stimulations that the outside brings?”
The answer to that question comes down to the degree of excellence of cat caretaking provided. The answer really is not whether the cat could or should be an indoor cat or an outdoor cat but something in between, a compromise which allows outdoor stimulation whilst ensuring that the cat is as safe as if she were inside. Committed cat caretakers are sometimes able to work out how to achieve that.