At 3.5 years mine was reading weak using a load tester. It was replaced at year end since it is a business expense, and I can not be inconvenienced by a dead battery trying to leave a hotel parking lot on the way to a morning meeting with a client.
Heat and vibration are the #1 death sentence for batteries, next is running them low and not properly charging them. Both of those diminish the batteries lifespan and then the cold weather / added amperage starts finishes them off (not counting parasitic draws, of course.)
Full disclosure, when I first went to buy my car, the battery was dead. they got it to run and promised me a new battery. Then they said that , since it was working, there was no need to change it. A few weeks ago, we had a cold spell and the car would not start. They swapped out the battery then. All good since then.
I do have a good Battery Minder battery charger. It is suppose to be able to desulfate a battery. It is possible that, putting a run down battery on it has brought them back to a usable state.
5 years on mine with 44,000 miles. I just swapped with new Motorcraft 590cca - $130 at Ford dealership
I was noticing it was struggling starting a little after sitting in cold for a few days. Also had a weird turn signal electrical problem that came a went, which I read could be due to low battery. New battery and starting feels stronger now, also didn't lose any presets on the radio.
I would replace if I were you.