Yes Overclocking will decrease the life of a GIVEN CPU, But very hard to qualitatively state how much.
I have an i5-2510K, let’s say that it will last 10 years if I do NOT overclock and treat it nicely.
I've Overclocked it to 4.2 (Very Mild), OK so I'll get maybe 6->8 years, If I went to 4.6 GHz maybe 5->6 yrs.
BUT in 5 yrs it is obsolete anyways.
Those that like to push the boundaries do NOT expect it to last beyond the time they will upgrade as the “NEW” kid on the block blows the doors off theirs anyway.
The problem here is that the failure point is beyond its useful life, SO WHY NOT.
PS I got the i5-2500k very shortly after it came out; it's been OCed since 30 days after I got it. While I'm still satisfied with its...