The Under 21 rule is very simple - for Euro qualifiers and finals, there is a date specified that they have to have been born on or after (for the 2009 Euros, this was 1st January 1986). If the rule was that once you reached 22, you couldn't play, you might get the prospect of someone being 22 the day before the Final.
Essentially, anyone eligible to play in the first qualifier, can play all the way through the competition.