On-line advice says residential gas water heaters last about 8 to 12 years. I have lived here for 12 years and my neighbor says my water heater was 5 to 10 years old when I bought the house. So the water heater may be 20 years old. It still works fine but I wonder if I should get it replaced while I have time to find a good plumber rather than waiting for it to fail and have to use the first available plumber? Thanks for any advice, especially from plumbers.