What causes spinal stenosis?

You can't: The size of the canal is inherited. The secondary degenerative leading to further narrowing is also linked to genetics. You may be able to reduce this by healthy living (regular exercise, no tobacco use, etc), but overall you have little influence on this condition.

Born with or develop: Spinal stenosis is a narrowing of the spinal canal leading to compression of nerve tissue. You can be born with it or you can develop it with age related or degenerative changes or acquire it through deformities/instabilities or with disc herniations, synovial cysts or after some spine fractures.

Spinal stenosis.: Spinal stenosis in a condition wherein tissue grows around the spinal cord, causing gradual compression and pain. It's progressive and unrelenting. Unfortunately leading to debility and pain. Surgical options are available which may help, but left alone it'll cause progressive limitations. The rate of progression is unpredictable.

Degeneration: The most common type is caused by a gradual, progressive arthritic process in the spine.