only some models can have cores unlocked. These are higher end models of a quad core that were downgraded to fit a lower price point to meet demand.
If you are lucky enough to get a CPU that can be unlocked, it shouldn't affect its life, since thats what it was designed to be.
OCing a CPU only really affects its lifespan w/ increased voltages and thus increased heat. For the average user, the lifespan decrease is almost negligible considering how ofter the turnover rate is for hardware these days.
almost no one keeps a CPU long enough to notice a decrease in life.