1) how do you determine a young player's developed stats?
On a player's profile page, next to his age will be displayed a percentage. This indicates how physically developed the player is relative to his eventual prime. Using that percentage, you can work out the math with the tennis balls on the player card for strength and speed. For endurance, however, the effect of age is doubled. I believe the formula you use to determine peak endurance is End/Age^2.
2) at the journeyman levels, I assume the intention is to keep form and fatigue optimal before taking part in tournaments? However, I find fatigue tends to potentially rise a lot faster. How important is fatigue from that perspective?
Fatigue is the backboard for the development of your young players. Players shed 50 fatigue per day, and to optimize their development, you do not want to waste any of that opportunity to shed fatigue. Of course, fatigue will rise more quickly when young players have less endurance--and that's okay. You're right that a player who enters a tournament with higher fatigue will likely suffer in later rounds, as fatigue >500 leads to penalties on their ability. But I find that to be less of a concern when the primary focus is on providing the players with experience, which is most reliably done through practice.
3) does it make sense to take a complete week off, ie not even a practice tournament and just do friendlies in order to get form and fatigue optimal?
I suppose it could, in the right circumstances. Brian had done a little experimentation to find that friendlies are only about 75% as effective as practice in providing experience relative to causing fatigue. But if you enter what would be a practice week with high fatigue, you could take this approach.
The game is supposed to hold your players out of practice matches when their fatigue is >300, but I have seen this rule broken before.