At what age will I experience significant physical impediment to doing intense (military grade) exercise often

Answer 1

Probably late 30s. Your bones, back and joints will not be able to withstand the training as they do when you are young. You should modify your exercise program to include low-impact exercises (swimming, cycling) to keep your bones and joints in good condition. Once you shred your knee, rupture a disk in your back, fracture a bone somewhere, it will be the beginning of the end. Stay healthy, take supplements, calcium, antioxidants.

Answer 2

Probably late 30s/early 40s.