In article .com, JB wrote:
My 40 gal gas water heater is about 13 yrs old. Water is hot and it
works fine. The only sign of age is exterior paint starting to flake
off with rust underneath. (Its kind of strange...the flake bits are
perfectly round...like tiny polka dots.)

What is the average life of a gas water heater? I don't want to
replace a perfectly good heater with lots of life left, but I don't
want to play Russian roulette and come home from work one day to a
swimming pool in my basement!

What are the signs that its time for it to go?

I remember this being asked in a newspaper column by someone saying he
had a 15 year old water heater. The answer in the newspaper column was,
as best as I remember, "15 years, TICK TICK TICK!"

As for a warning sign: Often, but not guaranteed, is that hot water
runs out much more quickly. What happens (at least sometimes) is that a
tube in the tank that helps the water heat up evenly falls out of place
due to corrosion, and this has the effect of simulating a much smaller
tank capacity. Whenever I noticed the water heater running out of hot
water easily because of this, the water heater leaked in a matter of
months.

- Don Klipstein )