It would appear crazy on paper, but it allows some consistency over the 2 year tournament period, rather than having to plan for players to drop out it they turn 21 after 3 or 4 qualification games, or in the middle of the final tournament.
Players born on or after Jan 1st 1994 are currently eligible to play Under 21 games and in this summers European U21's championship.
It is always taken from their age on the 1st of January the year the qualifying tournament started, the current qualification process started in 2015.