You should be able to loan your own players out. That way you could let a 19 year old get a season of experience points at another club if he's not good enough to play for your team yet.
I also think your top players should be able to put in transfer requests if they're not being played or another top team shows an interest in them. If they are unhappy then their fitness levels should decrease. You could then cash in and let them go to the other club, play them more to make them happy or increase their wages to make them happy, thus increasing their fitness levels.
I also think your top players should be able to put in transfer requests if they're not being played or another top team shows an interest in them. If they are unhappy then their fitness levels should decrease. You could then cash in and let them go to the other club, play them more to make them happy or increase their wages to make them happy, thus increasing their fitness levels.