U21歐國柸點解有球員過21歲?

I think you have misunderstood the point U21...

The players need not to be under 21 of age at this moment/ during the finals, but they are under 21 of age when the tournments start, ie: from the qualifying round.

U21 European Championship qualifying round actually started in 2005, which means, the players are eligible to play in the tournment if they are under 21 of age in 2005, ie, born on or after 1 Jan, 1984.

the document that mingyan228 provide is actually the regulation for the next tournment U21 Euro Championship 2009.

Here is the one about this tournment, ie, 2007
http://www.uefa.com/newsfiles/459787.pdf
And you may check in there under "Player Eligiblilty", and they clearly state that "Players who are eligible to take at the start of the Competition will be eligible until the end of the competition."

Hope this answer your question.

歐洲青年U21國家盃賽制


2007年 U21歐洲國家盃是由荷蘭主辦，共51個歐洲足協成員國參與今屆賽事，賽事分為初賽週及決賽週兩部份，身為主辦國的荷蘭將直接晉身決賽週最後八強分組賽。 每一支國家足球均可以委派不多於三名年齡不超過23歲的球員上陣比賽。
16支國家足球係數較低的球隊先進行兩回合主客制的外圍賽，8支晉級球隊連同其他34隊共42支球隊進行分組賽賽事。42支球隊將分成14個小組，每組3隊以小組雙循環得分制進行，每組的首名共14隊得以晉身附加賽。（若出現同分，則順序以同分球隊的對賽成績、得失球差、得球、作客入球、小組全部賽事的得失球差、得球及作客入球等決定出線席位）。
附加賽採取兩回合主客制，若兩回合相加比數相同，則以作客入球優惠計算，如果仍未分出勝負需加時，加時並不設黃金入球或銀球制，但計算作客入球優惠，若加時後仍然打成平手則以互射 十二碼 分勝負。
8支晉身決賽週的球隊將分成2組，以小組單循環得分制比賽，每組得分最高兩隊晉身四強（若出現同分，則順序以同分球隊的對賽成績、得失球差、得球、小組全部賽事的得失球差及得球等決定出線席位），由四強至決賽均採取一場決勝負的形式進行，若90分鐘內打成平手，需加時再決勝負，加時並不設黃金入球或銀球制，兩隊必須完成加時上、下半場各15分鐘的賽事，加時入球較多的一隊勝出，若加時後打成平手則以互射 十二碼 分勝負。