well dont give up on your 21 year old. even though he wont train as fast, if he hasnt capped on his training, you will still be able to train him. it will just take slower. I remember with some of my guys who were 23, 24, and doing like, 3 weeks of single position training to get those one or two extra pops in certain skills, and it just makes such a difference, especially the secondaries.