At what age are girls too old to be flower girls?
Flower girls are generally between four and eight years old. (Sometimes, "mature" two- and three-year-olds pull it off quite charmingly, possibly with parents escorting them down the aisle.) If the girl is a bit older, you might consider making her a junior bridesmaid -- junior bridesmaids wear dresses similar (or the same) in color and style as the bridesmaids', they walk down the aisle in the procession, and they stand at the altar or stage with the other attendants.
See More: Kids at Weddings , Choosing Wedding Attendants , Your Bridal Party