Dogs can be altered as young as six weeks old with no health/behavioral side effects. In fact, young puppies recover much more quickly from the anesthesia and the surgery itself than an adult dog does.

Having the sex hormones in the body while the dog is maturing results in the growth plates closing six to eight weeks earlier than in the altered dog. Some people speculate that this may predispose the animal to injury, but there isn't any solid evidence to back that up.

Waiting to spay a female until after her second heat takes away any protection you would have gotten from mammary cancer. Mammary cancer is the most common form of cancer in dogs, and around half of all mammary tumors are malignant. I think it's worthwhile, if you aren't planning on breeding the bitch, to spay before the second heat.

Plus, having seen three pyometras in the last two weeks, I'm all for ANYTHING that prevents that!