Youth national team competitions seems to differ in their age rules. The youth version of the UEFA European Championship is for players less than 21 years old, while in similar competitions, like in South America and specially in the FIFA World Cup, the limit is 20?
The answer to this is variation. Both organizations do not like overlapping. Another example of this is that the age rules in the Olympics for soccer is much different than the World Cup.
Without a forced rule on this, each region can decide upon their own age-group. The exact reasoning is not known by me as to why each region chose their own age. This may have something to do with prior youth teams in the area.