I'm not one of the younger either, but filling the hole will do no good, and may just make the matter worse. (Ask an arborist, and see if you get the same answer). The hole is likely due to rot (decay) to begin with, unless someone has bored out the hole with a tool. Probably a limb died and the decay entered the tree through the broken limb, rotting it out and making the hole.
Finding a way to keep the hole dry would be the best bet to keep it from decaying further. Possibly, the decay could be slowed a bit if borax was added to the water in the hole. Borax will prevent decay, and possibly will wick down into the wood with the rain water and "preserve" or "prevent" further decay.
It may take a long time (years) for the tree to rot enough to have to remove it. If it were mine, I would try the borax (20 mule team) and try to prolong the time.