Many clients often ask me how long their water heater should last. There are several ways to answer this question. First, the average life expectancy of a water heater is 10-12 years for a gas fired unit and slightly longer for an electric one. Some brands seem to last longer than others. Recent design changes to optimize efficiencies has shortened the life of the water heater. They started thinning the steel of the tank and the glass liner. When a water heater goes bad they start leaking water or start making gurgling noises due to hard water buildup on the bottom of the tank.
Manufacturers say you can increase the life of the unit by draining a cup of water every month from the tank drain. The drain is located on the bottom of the tank and usually has a water hose connection on it. Remember, the water is about 120°F and will burn you, so be careful. Draining a little water helps remove hard water minerals that settle at the bottom of the tank. I have found from inspecting homes that when a home has a working water softener, the water lasts much longer.
I suggest when a water heater gets older than its tank warranty and you have not been draining water as per the recommendations above and you do not have a water softener, you should watch it closely. Be proactive and replace it before it finally does reach the end of its useful life or you may get water where you do NOT want it. The Photo below is of a 20-year old gas fired water heater still working. The water softener probably helped extend the life.