You mean the system predefined overclock?

Normally that is just a very small overclocking with potential that the overclocking margin is far more than that.

If you done it manually, you can get closer to the OC limit of your hardware.

Theoretically yes, overclocking can shorten the life span of the hardware. But most of the parts being overclocked are simply far too durable that even under overclocking (provided you have a proper cooling on them), they can still last pretty much forever.