Depends on the volts.
It will lower the life expectancy, but unless you like to keep your cpus for 5+ years its not usually a problem. They usually don't just die either, they will simply require more volts to keep stable (even at stock) as they get old.
I don't think it will be proportional, so higher volts will reduce the lifetime more each time you raise the volts than lower volts.
Obviously really high volts will just fry it.