Im guessing that the service life in Kentucky is probably similar to East Tennessee, so here is what I find in my area.
Electric heat pumps last about 12 - 14 years.
Gas and propane forced air heaters last 16 - 20 years.
Split a/c units with gas heat will last about 16 years.
Obviously the life span can vary a lot depending on the use, maintenance, quality of the equipment in the first place, and just plain old luck.
The outside unit will usually have the compressor go out. Sometimes the inside coil will need to be replaced too.
The gas furnace will have the blower bearings go out, the burners and controls will wear out, and the heat exchanger will finally rust out. While you can replace the blower and burners and control parts easily, when the heat exchanger goes, its time to replace the entire unit.