A standard tank water heater usually lasts 8 to 12 years. Some make it to 15 with soft water and a replaced anode rod; some fail at 7.
The tank is steel with a thin glass lining. Every day, hot water flexes the steel, minerals settle on the bottom, and the lining picks up small cracks. A sacrificial anode rod inside the tank corrodes on purpose so the steel does not, and once the rod is used up, usually after 4 to 6 years, the tank itself starts to rust from the inside. That is why most tanks fail somewhere between year 8 and year 12, and why they almost always fail by leaking rather than by stopping.
Water quality is the biggest variable. Hard water leaves more sediment, which insulates the bottom of a gas tank and makes it overheat and rumble. Softened water is gentler on the lining but eats the anode rod faster. Well water with a low pH is hard on both. Gas and electric tanks have similar lifespans; electric tanks have no burner to foul but their lower heating element sits in the sediment.
The warranty tells you what the maker expects. A 6-year tank and a 12-year tank from the same brand are often the same tank with a bigger anode rod, or two rods, in the longer-warranty model. Treat the warranty length as a rough floor, not a prediction.
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