First of all. There is no guarantee that he will be 22 years old in season 20. It's only one from all possible scenarios, but it is NOT 100% sure.
And second thing. You can't handle the truth. Rules should be equal for all. There shouldn't exist specific cases, only because You have different opinions. For example in Tunisia player with potential = star is very strong candidate to play in U21. In fact, I remember time when in Tunisia exists only one club. So EVERY player could be 100% sure to play in U21, independent of his potential and skills ;-)
(because there are 12 slots in match lineup and from age = 18 to 21 we have 4 season and one club can gain in draft only 3 players per season - 4x3 = 12).
We can't force all managers to know differences between for example, Tunisia, Japan and Poland. And therefore we can't penalize manager only because he doesn't know that one U21 is stronger than another.