Ed, is your battery showing any signs of problems? I wouldn't replace it just because of age. There are a lot of factors such as duty cycle, temperature, condition of the charging system, battery temperature, etc. that contribute to the demise of a battery. You don't want to replace one too soon.
The trick is to replace the battery the day before it dies.
Check the terminals, clean them when needed, check the water level and wait until it gives warning signs like slower cranking in colder weather. If you're in doubt, have someone check it for you.