The furnace here is probably about 50 years old. However, it's probably not the oldest one that will be mentioned here, knowing the people on this site!
Although it's forced air electric, and I think those can probably be kept going more or less forever. This one certainly has been repaired, and is not original. How knows how many sequencers have been in the thing? (Note to those who don't know: sequencers turn the furnace on and off slowly, one element at a time. An electric furnace pulls a lot of power, and switching it all on/off at once would not be good.) The original design certainly used a different sequencer scheme where at least one element would start up before the blower did. (Now the blower is first on/last off.) The blower motor is also not original.