The spinal canal is the hollow tube formed by the bones of the spinal column. Anything that causes this bony tube to shrink can squeeze the nerves inside. As a result of many years of wear and tear on the parts of the spine, the tissues nearest the spinal canal sometimes press against the nerves.
This helps explain why lumbar spinal stenosis (stenosis of the low back) is a common cause of back problems in adults over 55 years old. Sometimes people are born with a narrow spinal canal. The aging process speeds up the start of any problems.
But usually it's wear and tear on the spine from aging and from repeated stresses and strains that cause many problems in the lumbar spine. As we age, the intervertebral disc begin to collapse. The space between each vertebrae starts to shrinks.
Bone spurs may form that stick into the spinal canal and reduce the space available for the spinal nerves. The ligaments that hold the vertebrae together get thicker and push into the spinal canal. All of these things together cause the spinal canal to narrow.