Growing up we always had indoor/outdoor or barn cats. I never saw a single one of them die of old age. In fact, I don't think any made it past 3 years old :sad: . The area we lived in had a high coyote population, which, although no bodies were found, I'm pretty sure accounted for most of the deaths. So I'd say indoors is better, unless you live in an area with no nearby roads or predators.
It's unfortunate though because I really do think outdoor cats are happier, so if a safe outdoor area can be made that is ideal, or using a long leash although I don't see how that would protect from predators, if anything it would make them an easier target.