Water heater safety isn't that simple.
Updated: Sep 20, 2019
Although a water heater tank is basically an overgrown Thermos bottle, we inspectors see common installation issues that can significantly reduce safety. Obviously, safety features won't be tested unless there is a failure in the system. Because the water heater is working fine right now doesn't mean it will behave when you most need it to.
Common safety issues include lack of or poorly installed expansion tanks (see expansion tank post elsewhere in plumbing tags) and strangely plumbed Temperature Pressure Relief valves. As the article linked to the above image describes, it is difficult to imagine what may happen if the TPR valve discharges (due to a failure in the thermostat). Think HUGE pressure cooker suddenly with a 3/4 inch hole in it. One cannot use PVC pipe for the TPR discharge pipe because PVC can't take the heat and these pipes should be six inches from the floor otherwise the super heated spray will either splash up too much or out too much.
Failures include the bottom rusting out and flooding the home and expansion tanks filling with water and rupturing pipes. Yet if the thermostat fails and the TPR valve fails you have a steam rocket. Manufactures recommend replacement very 8 to 12 years. Reasons for replacement include integrity of the tank, tightness of the seals to the plumbing system, and age of the fail safes (thermostat and TPR valve). Bottom line is safety. Plumbers recommend not keeping a water heater for more than 15 years.
Other problems caused by old water heaters include steel flakes from rusting tanks settling on your copper pipes and causing galvanic corrosion. I have seen whole lengths of horizontal copper pipes with green dots at the bottom, which eventually leak. Also, when water heaters become filled with scale deposits they become much less efficient in heating the water.
Here is a picture of a water heater with so many metal flakes that they came out in the tub!