Actually, I believe that most vets will not crop past 12 weeks - so the window of opportunity for cropping is really between 10-12 weeks of age. Beyond 12 weeks, cropped ears are unlikely ever to stand. Now there is the occassional puppy for whom that isn't true, and they have a particular type of ear cartillage such that vets may recommend cropping after 12 weeks. But it would be very rare, I think, for a puppy to be cropped beyond 14 weeks.
If you do a search of the forums, you'll find many previous threads on cropping, including the age at which it can be done.