I have always know it to be too old. A while back, someone was mentioning that their vet liked to wait and do it later. Something about cartlidge forming and getting better results...wish I could remember.
I was under the impression that cropping was done between 3 and 5 months so the cartilage was still not completely set yet. After that it was pointless since it was in the shape it would be for life. I would say 6 months is too old.