Here are my results with team rebounding training the since the week with 13 pops. I don't recall off hand now if that was my first or second week when that happened, but here is the layout from that point:
Week 1: +13
Week 2: +2
Week 3: +2
Week 4: 0
Week 5: +4 with a stamina drop
So that still averages out to 4 skill pops/week.
This with a minimal trainer. Obviously, younger guys develop faster. I'm moving to a different training this week, but I haven't decided which, yet. I may stick with IS to continue to build the interior game.
To your other question, obviously, younger guys develop faster in general. I seek younger, higher potential guys. My main objective now is to find a solid core of U21 guys as my starting point of trying to compete again, mainly through the draft as I generally don't have a lot of time to scour the transfer market for steals. Accordingly, there may be the odd PF/scorer that I keep to facilitate Cup runs until the 26-30 range.
I think there are 3 prime windows to sell a guy...1) their first year; 2) the 26-30 range, if you've managed arena and other business aspects wisely, you should be able to find a better, younger replacement develop them a little more, sell them for more and keep that cycle going; and 3) 32-33 when they still have value, for teams looking for economical veterans for promotion/playoff pushes.
I have $8.4M in the coffers, so once I have that youth core in place, I'd be looking for 1-2 pieces that I haven't been able to find/develop when I am ready to start competing. That may not be for another couple seasons. I'll probably sell most of the guys I won't keep by the time they are 26.