I have added when to cash in, and also a minor change to general, as general (although very minimally) does train keepers
Do alot of people not train till 21, might be worth mentioning that for balance. The bit on form about goalkeeping was there, just not very clear really. So I've tidyed it up so hopefully it is clear now that general trains goalkeeping and also that shooting trains set pieces
I feel personally that 21 is getting on a bit, so I put 20, but that said if the general consensus is 21 it can be changed
As team gets stronger, it becomes important to find the balance between trainable and playable players, therefore I'd say, that 21 y.o. are not too old for training.
With through passes, you can train 20 people, not 18. You could play twice 5-5-0.