To reach 100 TSP by 21 years, the youngster should start with at least 65 TSP and have 35 pops in 42 weeks, which is doable but very rare. A 18y old I monitor had 13 pops in his first 14 weeks, but that's because of high sublevels and starting DR=1 which trains very fast.
Anyhow, if this player is to reach 122 TSP by end of his 22 years, he should get 22 more pops in 28 weeks. I don't say it isn't doable, but it's possible only if he trains guard skills because 1 on 1 and passing are such fast trainings. If he would have to train inside skills before he is 23y old, he would either have to be at least 196 cm (in which case he wouldn't get 35 pops in first three seasons anyway) or have great elastic effect on IS which trains fastest of inside skills for small guys (for example, IS 4 and ID 9).