From what I know, not the overclock itself will shorten the life of your cpu or other components, but the high voltage and temperature will. Every cpu has a max "normal life" voltage. Even if your temps are low the high voltage will shake some days out of a cpu's life
pre-built PC's normally come with the vcore on "auto" this in turn makes the board apply much more voltage than needed and still the CPU's lifespan remains the same unless the CPU is not getting adequate cooling. So whoever is saying that adding voltage kills the lifespan needs to do some homework. Like I said as long as you keep the temps down you have nothing to worry about.
high voltage and high temps will eventually fry the CPU, which is contrary to what I am explaining above.