yes they will train faster as long as their age doesn't go over in the next tier. For example, if he turns 22 he will probably train slower because 18-21 is the same but there is a drop to 22 from my experience. Not sure what other tiers are, maybe 25, 28, 31?

Not sure about your 2nd question. I don't play friendlies and i try to play younger guys in the competition games and then i train everyone under the age of 30 to be at 97-99 condition for the next match.