From what I've seen of farm cats, indoor/outdoor pets, etc....
Most kittens will get killed somehow. My cousin tried her best to keep the farm cats healthy but she was lucky if 1 kitten from each litter lived. If they lived to be a year old, the females usually lived about 5 or 6 years. They all died after 8-10 litters, although it was never proven they died, they just disappeared right around the time their last litter was expected and never came back. So presumably they died while kittening. The toms usually disappeared after 2 or 3 years. Some of them would be seen wandering the fields occasionally but most were presumed dead. But that was unvaccinated, unaltered farm cats with minimal vet care (they'd get antibiotics if needed and de-wormer but that was it). They were well fed, had access to clean water, and dry warm shelter.
I prefer not to have cats that go outside. It's too stressful for me, I worry so. But I took in a cat from a friend. He came to her when they lived on a farm, probably a year or 2 at the time. About a year after that they moved to town and brought him with them. He was neutered and vaccinated soon after. A few years later, he came to live with my family. We had him tested for FIV/FeLV (negative), re-vaccinated, de-wormed, etc. He came and went as he pleased. He was NOT happy if forced to stay inside, eevn if it was because of the weather. He died last year, at the emergency vet's office, of an unknown illness. Possibly cancer. By our best reckoning he was around 14 years old. And the female we got at the same time is still with us, about 15 years old, but she doesn't go out anymore. My first kitties, who used to go out occasionally, are 20 1/2 years old now.
Alley Cat Allies estimates that most altered ferals who are part of a well-managed colony will live to be about 12 years old. This seems reasonable from my observations. Of course any cats living near a highway or another hazardous situation will face a higher chance of dying in an accident.