I just had Strider nuetered at 6 months, but I have had a doberman nuetered at four because A) He started marking territory and we disagreed that the livingroom was his territory, and B) I had to bring in his unspayed sister, and the day after I got her she went into heat and I couldn't get her spayed. My house was too small for a 110 pound sexually frustrated doberman! Both recoveries were uneventful, and although I wouldn't wait that long again, I never regretted doing it. Jules is right though, it was much more expensive to get it done on the larger, older dog. Our vet said you had to wait until 6 months on a male because if you went any earlier the hormones that control them muscling out could be affected, and you could end up with a bitchy looking dog. I assume that it's accurate.
Stella