It really depends on the circumstances and why you want to neuter the dog.
For example, if you have an intact female or two and another intact male of another breed, and three of their young adult mixed breed female offspring who are all intact as well, then you will want to neuter the boy, now, at 8 weeks. (Sorry, that was the scenario of some guy that got himself killed by his dogs getting into a fight and he and his wife were attacked trying to separate them.)
Really though, it does depend on the situation and the reason. If your reason for neutering is to prevent your dog from roaming the neighborhood in search of females to impregnate, go ahead and neuter at six months. Chances are slim that he can reproduce before them. This is a containment issue more than a hormone issue, but if females in heat are anywhere in the neighborhood an amorous young male may be more than ready to find a way out of the yard. Frankly, keeping a GSD at home and safe ought to be a priority regardless to whether or not he is intact.
If the reason for neutering is that young males might mark or hump or become pushy or stubborn in training -- most of these are training issues, but neutering may be helpful. Waiting for the growth plates to close is the best bet. Usually 12 to 18 months is fine. But lots of dogs are neutered younger, when they suddenly become teenagers. The butthead stage claims many a young dog's hormones.
If you want to neuter for health reasons, wait for him to be fully mature 2-3 years, or don't neuter at all -- far more healthy.
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