Is it unusual for someone in their mid-thirties to be diagnosed with spinal stenosis?

Not very unusual: Spinal Stenosis is a broad term. People in their thirties who have spinal stenosis usually have a disc herniation which is a different type of spinal stenosis than what older people have.

Somewhat: About 30 percent of people will get some degree of degeneration in the spine by the age of 35. It is less common in younger patients, but I have even seen people as young as 10 or 12 years old that have it.