First I would install a good fan & temp monitoring app. I like this one: Temperature Gauge Pro. It will give you the details on what is happening with your sensors and the fan it self.
The heat sink (the CPU's heat management) its self is mechanical, so your issues are limited to the three possibilities:
- The thermal paste was not applied correctly, missing or bad.
- The fan unit is not cooling the heat sinks fins because it has failed or it is fan blades are dirty or the fins are clogged with dust.
- The liquid within the heat transfer tube has leaked out (if it has liquid) so it is failing to conduct the heat away.
Given the age of your system it should be covered under warranty. I would strongly recommend you get your system into an Apple Store ASAP as you don't want to lose the free warranty repair (logic boards are quite expensive to replace).