There is no single expiration date stamped on an HVAC system, but there are honest ranges. In our Tennessee climate, a central air conditioner often lasts about 12 to 18 years, a heat pump about 12 to 16 years because it works in both summer and winter, and a gas furnace commonly 18 to 25 years since it only runs in the heating season. Treat those as planning ranges, not promises: two identical units on the same La Vergne street can age years apart.
Why the spread? A system in Lake Forest Estates installed around 2004 has now seen more than twenty Tennessee summers, long humid cooling seasons where the unit runs from May into September and sometimes beyond. A system near Percy Priest Lake also breathes damp lake air that is harder on outdoor coils. Meanwhile a well installed, well maintained unit in Woodland Hills can pass the top of its range still cooling fine. Age starts the conversation, but condition finishes it.
The useful question is rarely the birthday alone. It is age plus repair history plus how the system behaves now: cooling evenly, running reasonable cycles, and costing what it used to on summer bills. That combination is what our repair or replace guide weighs, and it is why we answer lifespan questions with ranges instead of a sales deadline.