The average water heater lifespan is 8 to 12 years for tank models and 15 to 20 years for tankless units. Gas heaters tend to last slightly less than electric due to combustion component wear. Water quality significantly impacts longevity with hard water shortening the lifespan. Annual maintenance including flushing and anode rod replacement extends life by several years. Check the serial number plate for the manufacturing date to determine your heater age.