I read an article on this site, written by Jos, on cryptorchidism, it says if they have not dropped during the first two weeks there could be problems. It was very informative, this is a link
A few days before birth is the minimum, about 6 years seems to be the maximum. There is no standard. Note that the term "dropped" is generally used to denote entered into the scrotum. They may be retained further up, and hence not dropped in the traditional sense, bu may descend later.
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