What is the expected life of a gas State Select Water Heater? Mine is 10 years old and I'm thinking of buying a repair/replacement warranty.

Expected life of most water heaters is about 10-12 years.

Depends on where you live, and where the water comes from. Also it might depend on if you have a water softener. In an area with lots of minerals in the water especially sodium, the heater will not last as long. Also a water softerner will leave salt deposits in your water and that is not good for a water heater.

Hard to say. How expensive is the warranty, and is it going to pay for the complete cost of replacing the water heater when it starts to leak, or is it going to pro-rate it. Ive seen water heater tanks leak within six months, and I've replaced ones that were more than 35 years old. I say, as long as its not leaking, and its still working, just wait till it fails and then install a new one. With the new one, you will most likley have between a five to ten year warranty without paying the extra cost of added insurance. Beware purchasing a warranty on a water heater as old as yours(read the fine print), because it most likely won't pay off in the long run.