relative to older players, yes, but some 18yos will train much faster than others.

Back to the original poster's point about one player gaining more from practice matches and then less than the other two on cardio/stretching and who's the fastest trainer, its all variance, players don't seem to get the same figure over and over for stretching or cardio/practice. Sometimes the player can have full condition and get +55 for cardio on hard intensity, then next time you train him on cardio, he gets +32 despite being the same quality, having the same condition etc. So it seems you can do the same thing over and over and gain different results.

What you wanna do to uncover the underlying training speeds is take get the players to the same point, so say exactly Quality xx, then balance their condition levels so they're equal and then record the results (condition loss as well as gain) of doing 5 or 10 sessions of each type of training. Then you take averages which will give you a far greater understanding of who will consistently train well.

In your example, it looks like player A just had a great training session in the practice match but if you give him 10 practice matches, all with same condition and intensity, he might not get near that level of gain again