As far as training guards, I have 3 19yo trainees, a 23 yo I still train, and a 22 yo I get half training minutes per week usually.
As far as I can see, the 23 yo doesn't train TOO MUCH slower, it's still a bit slower, but only by a small fraction of a level per week, perhaps 0.15-0.2 skillpoints a week. A big difference in the long run, but for short term, it's not at all bad.