Trainer is more important than youth trainer no matter the age of players you train, youth trainer just provides a boost in what main trainer already gives you. So as you understand lvl 1 trainer with lvl 6 youth trainer even for 18-19 yo players will not be effective. I suggest you get at least lvl 4 main trainer but since you are traning for your U21 national team if you can afford a lvl 5 or a lvl 6 they would produce better results for you. You should also contact your country's U21 coach for a better understanding of training and which are the most suitable training regimens for the players you are currently training.