What does it mean to divide the national team into U20, U2 1 and so on?

U 17 is to participate in the Asian Youth Championship. If you get the top four in Asia, you can participate in the World Youth Championship (and you can get a ticket for the World Youth Championship); U 19 is to participate in the Asian Youth Championship, before the Asian Youth Championship, 4) to participate in the World Youth Championship; U2 1 Basically, it is the Olympic team, participating in the qualifiers and participating in the top four of the Olympic Games. Other continents are similar. By analogy, some players of U- 19 and U-2 1 can also participate in the Olympic Games if they become members of the Olympic team. At present, no players under 17 have participated in the Olympic football match. The age of Olympic football match is 23, but at most three players are allowed to be over 23, and the rest must be under 23. This is the rule that it is difficult for many world superstars to participate in Olympic football, because many superstars are over 23 years old when they become famous.