Flush a gas powered water heater, not sure if it's ever been done in 15 years. Will it survive a flush?
Fifteen years is a pretty good lifespan for a water heater, so it could easily fail at any time -- especially if it hasn't been serviced regularly during its life.  Personally, I would prefer to be there working on it when it fails so that you can take care of it immediately (as opposed to, say, being away for the weekend while it's filling up your basement).

Before you start flushing, you should know how to turn off both the water and the gas to it.  You should already have some way of taking care of the water as part of the normal flushing process; a wet/dry vacuum will help clean up if you run into problems during the flush.  Since it's gas-powered, you should also have the number of a licensed professional who can install a replacement heater if the worst does happen.