Copied and pasted from AAA
Heat facilitates the chemical reaction car batteries use to generate electricity, but it also increases the rate of battery degradation. In cooler northern climates, a battery may last five years or longer, but in hot southern locales, a car battery will typically last approximately three years.
Batteries reside in a harsh under-the-hood environment where temperatures can easily exceed 200 degrees Fahrenheit in hot weather. To combat this heat, automakers may mount the battery in an isolated area, install a heat shield over the battery, or relocate the battery outside the engine compartment, often under the back seat or trunk floor.
I would guess that the battery that I replaced was 3.5 years old.