Oldest CPU I know still chugging at stock is my old Pentium 4 530 in a Dell serving as my parents computer, got that in summer 2004.
The only CPU I know that actually died was my old E6600 (the original Conroe, not the Pentium) I got in August 2006. It spent the majority of its life OCed to 3GHz @ 1.38v, spent a lot of time gaming and doing SETI and Rosetta crunching, and later as a MythTV DVR and Minecraft server. Was never pushed thermally. It went this April when it stopped recognizing 23 as a prime number. These are all accounting that my computers are on >95%.