Juveniles

There isn't an easy answer for this question, as there are many factors involved. Firstly, you need to know whether you want to sell him for as much money as you can or if you want to train him in your senior team. If you want to sell him, it is usually better to promote him as soon as he has turned 17 and if he has few skill sub-levels (so as to avoid having wasted weeks of training). But we will focus on the case in which you want him for your team or the national team. In this case, the age isn't that important. The following factors are more relevant:

There are only two things about the match itself which we care about. The only thing which matters about match reports is if a player has done any special event which depends on an speciality, so as to know that the player has this speciality. Player stars, however, are important. Stamina and form don't change in young players, but their performances may change a bit, mainly if there's much difference between their current skill level and their potential. Nevertheless, this variation isn't really big, so stars are great for knowing a player's level in each skill.

Before each match, you should follow these steps:
 

Don't care about match results. Not at all. The only thing that matters is developing the best young players you can. In fact, with the new reports, lineups with only nine players can be considered the best.