I replaced the battery on my '99 Focus when it was 10 years old just because it was 10 years old - it still started the car.
The battery on my current '03 Focus failed when it was 12 years old.
Clearly well outside warranty time!
Nowadays, cars charge so well, and start so easily, that we tend to think our batteries are fine when they actually only have a tiny bit of the original capacity left. It then only takes a small drop in temperature, or a short time parked with a light on, for it to have zero capacity left.
Chris