Each water heater manufacturer will recommend an annual tank flush of their water heater. while recommended, we stray from the path. We don't believe in the state of California due to our drought conditions that we should be draining 40- 50 gallons of potable water down the drain.
Each home is designed for a size/gallon capacity to service the appropriate amount of fixtures and bathrooms in the home. Your standard house has 2-3 bathrooms and a 40 or 50 gallon capacity water heater. You should not go down in water heater size if you have a 40-50 gallon water heater. The cost difference to go smaller does not facilitate any true savings.
Yes. The uniform plumbing code book and the California state plumbing code book are reprinted every two years with a small amount of changes. If you water heater has lasted the standard 10-15 years that means it has gone through multiple code changes. We recommend having a licensed contractor replace your water heater. Understanding the most current plumbing code ensures the new water heater installation is installed correctly.
In general, residential water heaters has lasted the standard 8-12 years.