The training speed is bracketed. It is true it slows as they age, but it falls in brackets, it is not gradual. I was under the impression though that it was at 21 that it slows down, because that is the age they turn from a youngster into a hot prospect. They slow even more when they go from a hot prospect to developed, which is at 24, and then after that the training more or less levels out somewhat
Well, that's not exactly what we can see in my screenshots
And I encourage you all to post your screenshots with same situations so that we have more evidence!