at what age can a german shepard   

Very ideal age for mating is not less than 18 months.
Good luck with your German Shepherds!

I would wait till they are 2 years of age. They need to stop growing first. Why are you breeding? Do you know the history of the pooch you are breeding your dog to? Make sure you have proof of x-rays for the hips to avoid problems for the pups. German Shepherds are prone to hip problems. I used to be a German Shepherd breeder and it took me a long time to find the right mate for my pooch. I did a lot of research, checked the bloodlines to make sure they were not related. Breeding should only be done to improve the breed.

Most pets are neutered since there are plenty of pets to have adopted out already. If the German Shepherd is purebred and you are thinking of making some money then being a boy it might not matter as long as he is physically capable. The female dog though should be old enough so that having puppies does not overstress a young dog.

Not sure what your situation is so hope you have a veterinarian that you can professional advice from. Or look it up on the internet or read a book from a library. Hope any of this is a help.