From what I was told and from what I've read, stenosis takes
several years, close to 10yrs to form and usually is from trauma
of the area.

When I hurt my back and they found I had a herniation at L4-5,
I had stenosis at L3-S1 already.  They did a lamectomy at L4-5
while they were in there, which seemed to help.

It took me almost 4yrs after my surgery to figure out how I could
have hurt my back growing up since I was 25 when I had my surgery.
I finally remembered that at camp one winter, I'd fallen off a bunkbed,
flat on my back.  I only remember it knocking the wind out of me, no
back pain at that time.  Came back to haunt though.

HTH