It is 5 years old - the top 1/2 stopped cooling, but the bottom half continued to freeze. In fact, a bag of ice I had on the top shelf thawed and as it leaked, it formed icicles on the bottom shelves!

Good day,
Sounds as if the fan in the freezer compartment isn't running. If so, remove the rear panel of the freezer compartment, inspect, and replace the motor if it tests bad.

If the fan is running, then look for a frost buildup on the lower half of the back panel.
If that's the case, then the defrost system has failed.
It is made up of 3 parts. The main heater, a calrod heater, mounted to the bottom of the cooling coil, and in the same area, a defrost thermostat, a round approx. 1" in diameter item that is clipped to the top of the coil.
Both can be tested with an ohm meter for continuity, once you remove the rear panel. However, the thermostat must be tested before it opens on it's own, which is about 50 degrees.
The last item is the defrost control which can be a timer or a solid state control board.
If it is a timer, you can advance it's shaft clockwise until the machine shuts off. If good it will turn the machine back on within 45 minutes. If not, it is bad and must be replaced.

Without a model number the answers are the best that be given.
If you want to post a response with details and model number, it would be a more detailed response from us.

Below is a description on defrost system basic functions to help you if needed.

http://www.appliance411.com/faq/howdefrostworks.shtml