Disc cartilage is subject to wear and tear, the same as hip and knee cartilage. The soft, moist center of the disc tends to dry out with age and narrowing between vertebrae occurs. Calcium deposits form along the edges of discs, probably to repair small torn places.
The disc can also bulge around the edges, similar to a under-inflated tire, All of this can be considered normal aging processes. In fact, MRI scans on people without any back pain at all may shown this and even HNP in about 20%. A diagnosis on imaging of disc degeneration is usually no cause for concern.