I've always been told the average lifespan of a computer is about 10 years with some parts being less and some parts being a little more. I have an old AMD K6 450 system at my office that still runs, it's overclocked to 500 an it even has an original 3dfx Monster card in it. I think we built that PC back in 1998 or 1999, Lara Croft was all the rage in 3D.
I pretty much overclock everything and when I upgrade I sell my old parts to other overclockers (usually) and I haven't heard of any of my old parts just dying. They've probably been resold or thrown away as obsolete.
The biggest thing about overclocking is Heat. Overclocking produces it and controlling it is what it's all about. At an idle you want your temps lower than a stock cpu and the same at full load. If you can do that then your cpu will last longer than you want to use it.
^^The same thing^^ applies to all your other components. Your mobo will run hotter when your overclocking, you have to cool it down. If you can do that then it will last just as long as it would at stock speeds.
When it's all said and done the big word in the term "The Average Lifespan" is the word average. When they make a 1000 components some of them will work for 3-5 years and some of them will work for 10-12 years and some of them will be even more or less. So you could "not overclock" and still have parts dying sooner than a system that is overclocked and vise versa.
Parts designed for overclocking are usually made to a higher tolerances and they cost more because of it. But they also tend to last longer.