Like most equipment, actual life depends on initial quality of the equipment, installation quality, and maintenance quality. For pool heaters the burner tube bundle is the most expensive component and replacement parts plus labor is cost prohibitive unless you DIY it and this is beyond your typical DIYer. With good attention to water balance and maintenance they can last 15 years or more. Other components are cost effective to repair if the tech really knows what he's doing. These are usually sensors or a board replace that cost under $500 labor plus parts. My pool heater is in it's 8th year with no indications of tube failure and required extensive electronic repairs from a lightning strike plus self-inflicted damage during the repair. Setting aside the lightning strike, it appears to be on it's way to 10 years or more. My neighbor doesn't use TFP methods and his pool service I know to be deficient. He replaced his after 4 years due to a tube bundle failure. Same heater, same water, same weather.
Chris