This isn’t normal condition and quite unusual since the pipe is hot with the burner turned off. A 15 years old water heater probably has a pilot that stays on all the time but that shouldn’t bring the flue temperature so high that you can’t touch it.
It should be barely warmer or the same as the garage’s air temperature. When the burner ignites it sometimes takes several seconds, maybe a minute if it’s cold outside, for the exhaust gases to create a draft and for the vent pipe to cool down (it might be hot for that short period of time).
After that, if there are no problems with combustion and the water heater is venting properly the pipe stays warm and you should be able to put your hand on it.
There are a few possibilities;
There’s something inside the flue (chimney) blocking it and preventing exhaust gasses from being discharged quickly to exterior.
The water heater has extremely low efficiency which causes a lot of gas being wasted and burned, thus increasing the temperature of the flue. Even with that it should cool down with the burner being turned off.
The baffle inside the water heater has corroded and dropped down onto the burner compromising proper combustion and venting (you can turn down the thermostat to minimum, unscrew / slide to the side draft hood (you may need to take apart section of the vent pipe) and check if the baffle is still intact and hanging inside the flue-way. Check this post - water heater maintenance and scroll down to where there are three pictures in one line to see an example.
Assuming that your furnace is installed right next to the water heater and sharing the same vent connector as the water heater, it might be causing its temperature to increase significantly. Furnaces, especially older ones, waste a lot of gas / energy due to their lower efficiency and since they usually have higher BTU output than the water heaters, their vent pipe temperature is much higher (it will burn your skin).
There’s also a chance that the furnace’s vent pipe has been improperly connected to the common section (shared with water heater) – more here in paragraph #6 – furnace water heater venting.
Please let me know after you do some “investigation". Just click “comment” below.