Has anyone diagnosed "lateral recess spinal stenosis" in anyone under 25 yrs of age? How common is lrss in this age cohort?

Yes and not common. There are other issues that can lead to this like spondylolithesis which is found in 6% of the population and significant repetitive loading as with football can accelerate degenerative changes in the neck and back leading to this as well. Obesity and smoking will also accelerate degenerative changes .