Unfortunately, age does matter. Subjectively speaking, it seems like right around 28 is when the injuries start happening. I don't know what you have to do physically to avoid this as my hamstrings started pulling and stretching like worn rubber-bands around 28, but I didn't have the kind of physiological knowledge (or staff with the physiological knowledge) to avoid it. The best thing this young man can do is:
1. He needs to be personally offended if he misses an off-season workout with Fields. Heck I would even rent an apartment in Atlanta if I had to- anytime that QB wants someone to throw to, be like- "Let's go man, I'll meet you in 10 minutes."
2. Get a sprint coach and train with the track college track team. 100m, 200m, whatever workouts (and stretching) those guys are doing, that's what he should be doing.
3. Keep those hamstrings loose. Man hamstring injuries are almost as bad as knee injuries if your game is speed. Hamstring injuries are deceptive, because you can feel great at 80 or even 90 percent, and then when you try to burst you will re-injure it.