As cats age, their nails tend to start overgrowing. They grow long and thickened and don't shed the outer sheath off. This can cause the nails to extend out from the ends of the toes and look like they're not retracting. When the nails get that long, they are no good for defense anymore. If the cat is limping, that is also a sign that the cat is in pain. The claws can get so long that they curl around and pierce the pad of the foot.

Most vets are pretty good at clipping them.