The flight feathers, those on the wing tip, are the first permanant feathers the chicken produces so as soon as they're in, you could trim them. You only need to trim the outer feathers tips on one wing, not both and make sure you don't cut into the quill, or solid part of the feather's rib that contains the blood supply to the feather.
Not all young birds are good flyers and may not need the trim treatment. You might wait and see which birds can fly up to the fence's height before you clip.