I still train my Omar Krasnić (24) and I'm pretty satisfied with results... I started training him when he was 22, and I didn't noticed some big difference between him and my 19 year old trainees...
And he had 8 Skill-Ups in last 12 weeks, so I don't see the 'one skill-up per 4 weeks' theory to be true... He would have even more skill-ups if I trained One on One or something similar, but I mostly trained Outside Defense...