within reasonable limits voltage regulation is primarily because of heat concerns. Max voltage is different for different processors (and sometimes diff. stepping in the same processor).
The more you over volt something, the shorter its lifespan will be. For the most part, the decrease in lifespan is negligible, because you don't keep the cpu for that long.
CPU's Memory, and Video Cards all have a certain voltage that you shouldn't go over for 24/7 overclocking. Depending on your cooling and the specif chip, its a different voltage