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.