No players decrease in skills besides stamina until they are 33+ years old so just because you arent training them doesn't mean they get worse. As for the training of your... kind of young players.. You just need to set the training however you like sometime during the week and make sure your trainees get 48+ minutes for the positions you set as being trained. On Friday mornings the training will update and what ever you had set for training will be trained and a player may pop in a category that involves what you were training.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs