For me, at level 5 I try to get 10m+ Nordgens....the last one I took a punt on was slightly below that. I can't say its lowered but I can say it varies hugely and sometimes its enough for my player to get more on Cardio than he does on Match Practice (but then I tend to be grateful for the huge Cardio result.)
I'm afraid I've not been around long enough to contribute anything on 22 yo or more fast trainers but from what I've seen so far there's no difference between the training rates of comparable (value) 18 yo or 21 yo ie within an age bracket - its all about the star rating.