1. What's the minimal age kids start playing competative basketball? I mean in an orgenized league and all. Do they have special rules? regular size baskets and balls?
2. Do you have full stats in youth leagues?
3. Do you need a special diploma to be qualified to coach in your country?
4. Usually when a kid is much taller/stronger/more athletic than other kids his age in his position, does he train and play with an older team, or his own age, or both??
I'll be the first to answer. This is how it is in Israel:
1. The leagues start from 5th grade (10 years old). 5th and 6th graders play in a special league called "mini-basket". they play 4 on 4, the rim is only 2.60m high (8'7''), no 3 pts, the games are 5min X 6 "quarters" and each player allowed to play only 3 of them MAX. They play with a size 5 ball.
7th and 8th graders play with regular rules and rim hight but with a size 6 ball. 9-12 grade (ages 15-18) play with size 7 ball and other regular FIBA rules.
Zone defense is allowed only in the U-18 league (11-12 grades).
2. The only formal stat is the score. you can barely know how much points scored each player. Obviously no stats on the internet. On the official site you can check the only the final score, which is also updated only after a couple of months. Sad but all true

3. You must go through a month long course to get a "basketball-instructor" diploma to coach kids from 5 to 16 years old. To coach 17-18 y/o and adults you need a "basketbal coach" diploma, which is a much longer course.
4. Very rarely a kid like that will play with older kids, where he'll realy develop his skills and not only depend on his phisical atributes. Coaches want to win and "milk" the kis as much as possible.