Salary and potential should be factors in who you train. I'd rather invest training dollars in a player with high skills (salary) who has the potential to be trained.
Age (18 y.o. Fast training, 19 y.o. Slower training), potential (How long you can trian a player), and Height (affects training speed) are the big factors of training. The younger the player, the faster the trianing, better potential= More time to train=better stats=Higher salary. Height helps training (a 7'4 Center will train fasted in ID than a 5'9 center). Salary is not a factor in training. I don't really know why U21 NT players needa good starting Salary. I look for Height and Potential. Salary will raise with training. U21 NT player will most likely have a higher salary, since tehy need good starting stats.