There have been many studies done by veterinarians, veterinarian colleges, shelters, etc. that show there are no adverse effects to early alteration. Putting an 8 week old puppy under anesthetic is no different than putting an 8 month old puppy under anesthetic. There is nothing about the youth of the dog that makes anesthetic worse for its body. There are also many studies that have shown there is no difference in adult size, height, weight, coat, etc. between a dog altered before or after sexual maturity.

In a perfect world, you could alter your dog whenever you felt like it, because in a perfect world, accidents would never happen. But, we all know this is not the case. Some dogs come into heat earlier than others. One female may come into heat at seven months, and another at five months. Also, some dogs have what is called "silent heat", especially their first heat. This is heat with not outward noticable symptoms, i.e., bleeding, swelling, etc. So, trying to judge when to alter or when to be "extra careful" based on first heat is a crap shoot at best. Also, in a perfect world, dogs never jump over fences, dig under fences, slip their collars on walks, jump out of window screens, sneak out of open doors when the pizza guy comes, etc. But, we all know this happens, to even the best dog owners. If you have an unaltered, six month old dog that does this, you could quite possibly be looking at a litter of puppies. And what the world does not need is more puppies, when thousands of dogs die at shelters every day.

My opinion is, there is no medical or cosmetic reason not to alter puppies at a young age. Progressive veterinarians and shelters have been doing this for decades, with tens of thousands of dogs, and no ill-effects have ever been proven. The reasons to alter early, namely less dogs in an already far too overcrowded doggie world, make early alteration the smartest and most human solution.