This past weekend I had the honor of marrying my brother (as in performing the ceremony) out in Chantilly VA, which is a suburb of D.C. Chantilly is part of the Arlington, VA Diocese. My sister was going to serve at the Wedding Mass until we learned that Arlington doesn't allow girl altar servers.
So where does the Church stand on this issue?
The duty of serving at the altar has long been one that the Church has thought of as fostering priestly vocations. A letter in 1994 (click here to read the letter, and here to read the 2001 follow-up) from Rome was the first to allow the possibility of having girls serve at the altar. Nonetheless, the letter did not require bishops to allow girls to serve.
Rome has given bishops the option of allowing girls to serve, although Rome does not require it of any bishop. Even if a bishop allows it in his diocese, it is still up to the pastor to allow girls to serve in each individual parish. If a bishop does not allow girls to serve, then that decision can not be adjusted by a priest within that diocese.
So a girl wishing to serve at Mass has two hurdles to clear. If her bishop is okay with it then her pastor needs to also be okay with it.
Why would a pastor not require it? Wouldn't it help foster religious vocations among women?
First of all, about the fostering of religious vocations - religious sisters do not serve in the sanctuary, where as priests do, so letting a young girl serve does not give her a taste of what religious life would be like for her, where as it does give a young man a taste of what religious life would be like for him.
Secondly, when you allow girls to serve, the guys who might be willing to serve usually scatter like bugs under a light. Girls are MUCH better at serving than the guys, are more confident, and guys only approach things that feel safe to them. For this reason, some parishes and bishops foster a "guy only" approach to servers. For the record, we at Ritter have adopted a "guy only" approach to our altar servers in hopes of fostering priestly vocations among our young men. Our young ladies at Ritter still serve as readers, gift bearers, and extraordinary ministers of Holy Communion.