Our furnace was over 42 years old when we replaced it this past spring. A good unit, at least old ones, lasted a long time. Of course we have two gas fireplaces that supplemented it for the past 30 years, so it didn't have to work as hard as a stand alone unit might. We had the thermocouple replaced a few times, had the gas lines cleaned and such, but the welds throughout the unit were starting to fail, so time was up, and we wanted AC.
If we are lucky the new one will last 10 years...