I don't think that flower girl dresses have to be identical in style, especially when the girls are not of the same age. Older girls will look more elegant in ankle length or tea length flower girl dresses, whereas, younger girls would be more appropriate and functional in shorter knee length flower girl dresses. However, it's important that the flower girl dress should match, as close as possible, to each other in color. It is also as important that the flower girl dresses should match the color of the bridesmaids dresses.