I have a Catholic Answer book from 1990. In it it states that altar girls are strictly forbidden. What is the final decree on this subject?
There is no “final decree.” As of the copyright date in your book, it was true that girls were not allowed to serve at the altar during Mass. Later, John Paul II relaxed that discipline to allow bishops to give permission for altar girls in their dioceses. Since it is a discipline and not a doctrine, Benedict XVI or another pope could change that.