I agree with the others, find an experienced vet, that has the best interest of your happiness with the job, and not the money to be made from the crop. I would be honest with them, and let them know that you are aware that he is on the older side of being cropped. Tell them that you only want them done if they feel confident that they are going to stand.
With that being said, my boy turned 14 weeks the day after he had his ears cropped. The vet was experienced with Doberman crops, and had to see him for a consultation before doing the procedure. We got a mediumish crop, although it looks long now, he has a lot of growing left to do. My vet doesn't like to do Doberman's past 14 weeks, and will do them only after a consultation. She said he had good leather, and great ear strength. :biggrin55:
Here is a picture they took for me when he went in one week after he had his crop done for a re-taping. He is probably the exception to the rule.
Good luck, and be sure to let us know what you decide! Oh, and I was worried about having it done, too, but am so glad now that we went ahead and did it.