Some of the factors that affect furnace age are weather, mileages, the amount of air conditioning used and how much heat is needed. The more usage of air conditioning, the more often a furnace needs to be serviced. The best furnace service is one that is done every three years. Older furnaces need some maintenance due to the normal wear and tear from running in the same manner each time it was plugged in. These types of older furnaces can last anywhere from ten to fifteen years depending on what the owner has done to the unit in the past.
Each furnace has different furnace ages. Most household furnaces will not have any age built into them. There are some that will need to be taken out and cleaned before they will make any sense. An example of this would be if a furnace takes a lot of air conditioning to run. Taking it out and having it cleaned will allow the heat to flow more efficiently, which will help it run more efficiently. This will also help with the temperature control of the unit.
The actual furnace needs to be replaced once a year. The older the furnace, the longer it will take to get an inspection done and some kind of estimate. The reasons for this vary from furnace to furnace. The main reason is the older the furnace, the more likely it is to have trouble. If the new model is not made for the furnace that has been used before, then the warranty may not cover the repair or replacement.