I would be careful about using that stated outdoor-only (ie, feral) cat lifespan for an indoor/outdoor cat, though the dangers are real. Ours is currently mostly indoor only (until we get all the cat-toxic plants in our yard removed) but we have neighbors and friends with indoor/outdoor cats that are definitely older than 10 years. Mitigate them with reflective collars and nextdoor or other neighborhood alert system to know when coyotes are around.

An indoor only cat never really grows up psychologically, you're always its ever-present mommy. I'm not sure I want that for my kitten.