Battery life depends upon a lot of variables but 5+ years of use is not unusual. If it still works well why replace it? When the engine starts to crank over a bit more slowly than usual when cold it's a sign that the battery is getting weaker and won't last too much longer (assuming your starter is good). Otherwise I'd ignore the "bad" reading Autozone gave you and just use it until the car gets to be hard to start..

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold