A properly installed furnace, given the correct ductwork design & properly sized ductwork including cold air return and supply air diffusers rated for the amount of air that the furnace you have delivers could last longer than 23 years for sure. But many variables can cause that furnace life to deteriorate faster than you'd expect. For instance a plugged and dirty air filter could cause the furnace to overheat because the blower would be unable to deliver the same amount of air required to remove the heat that the burners produce, therefore the relationship betweeen the combustion chamber wall temperature and the airflow would inversly change causing a change in overall combustion chamber temperature. The chambers are made of metal, that metal heats and expands at a certain expansion rate, if you change the airflow to temperature relationship drastically the metal would expand and contract at too rapid and excessive a rate, resulting in premature combustion chamber failure. I'm in the heating and cooling business in northern california and I have seen brand new furnaces less than a few years old with burned out combustion chambers due to improperly sized ductork and dirty air filters over and over. 23 years from an old furnace is about average-after that the energy loss and consumption is worth a change.