They also tend to have a greater need for such structured activities than girls who are usually more mature and responsible at this stage of life.
As a result, some parishes have found that the introduction of girl servers has led to a sharp drop-off of boys offering to serve. Once the boys have left and enter the years of puberty, it is difficult to bring them back.
Some pastors say this phenomenon is less marked where serving at Mass forms part of a wider Catholic structure, such as a school, or when siblings serve together.
It is also true that groups of boy servers have fostered vocations to the priesthood. But to be fair, this usually happens within a broader culture of openness to a vocation in which other elements come into play, such as the example and spiritual guidance given by good priests, and family support.
If, for example, a long-established program of boy servers has proved successful in promoting vocations or has been useful in helping boys avoid bad company and maintain the state of grace, then the good of souls obliges pastors to weigh heavily the spiritual risks involved in abandoning it.
When girls do serve, it is probably best to aim for a mixture of boys and girls — if only to avoid giving the impression to the congregation that Catholicism is above all a female activity. On some occasions, however, it might be best to separate boys and girls into different groups.
It is very difficult to lay down precise rules in a matter like this since the situation may vary widely between parishes. And it is not unknown to have sharp differences among the faithful who assist at different Masses at the same parish.”
February 17, 2004 follow up statement:
“Serving at Mass, unlike the Catholic’s right to assist at Mass and receive Communion, is a privilege and in some cases a vocation. But it can never be called a right. Therefore, I repeat that no one has a right to do so and to frame the question in these terms is to use political categories to seek to demand what can only be humbly accepted. “
Full text
here.