mmm... I will try to explain.
My old Q6600 2.4@3.4 (50% OC) is still functioning and it is 7 years old, I work with it rendering, playing and using it all the days for at last 12 hours at day for 7years. A CPU has a long live.
The OC is dangerous if you push too high the Vcore and have too high temps (90º+ C). But the CPU is not alt 100% all the day... when you are working with Office the CPU is doing... nothing (idle) wen you are playing the CPU is not at 100% (temps are not too high) whith this kind of life a overclocked CPU can live a lot of years.
CPU degrade in time because the electron migration caused by the higer temps. But if you try not to push much more than 75ºC when it is at full capacity, your CPU will be obsolet before it fails, like my old Q6600 (still working and helping me in render farm). Normal lifetime of a CPU I think is 20 years, with moderate OC only a little less lifetime.
I hope this helps... I´m not english speaker... sorry :S