I have a gas furnace that was manufactured and installed in 1988. I have not had any problems with it yet, but my HVAC guy says that I should look into getting a new one soon. I intention is to wait until my current one breaks down...
As a general rule of thumb 20 to 25 years is a pretty good run on a 80% furnace. However 90% furnaces you should be able to get 30 plus years out of pretty easy. Just keep in mind that maintenance practices are key to the longevity of any piece of equipment.
Character is doing the right thing even when no one is looking. Intelligence is understanding and practicing the meaning of character.
And of course, if you have an 80% furnace now, and it's getting a little beat up or has not been maintained as well as it could have been, and you have the money available, installing a 90+% furnace will start saving you money every month.... the sooner the better!