While not all breeds of dogs are the same, there is a growing body of knowledge that later, MUCH later is better. The Rottweiler community was approached some 20+ years ago, by a researcher from the Gerald R Murray Cancer Center, Dr. David Waters. They were looking to study longevity using Rottweilers as a model. The very first thing that jumped out was that hormones play the biggest part in female longevity. And in their findings extraordinary longevity could matched into 3 age groupings for females: a) females spayed under 2 years had x chance of reaching 13 years of age; b) females spayed between 2 and 6 years had 2x chance of making it to 13, and c) those spayed over 6 had 3 x chance! They weren't looking for that result, but that was the first thing to literally leap from their collected info. In talking with Dr. Waters one of the things that he says has happened is that researchers often note that a dog was spayed or neutered but not WHEN, and that has skewed a lot of earlier research. There are also a fair amount of issues that can be easily caught and diagnosed and treated that do not outweigh the benefits of hormones.
Anyhow as breeds do vary in terms of longevity and what diseases are prevalent, your breeder should be able to help you with this decision-making process. We do have to realize though that early spay/neuter is a medical response to irresponsibility in pet owners who fail to keep their dogs from conceiving and dump their problem on the general public. Most veterinarians do not believe that their clients have the ability to keep their girls from getting pregnant. That says more about us than the benefits to our pets. Personally, we ask in our pet contracts for spays after 2 years, which allows permanent OFA Hip x-rays to be done at the same time. Dr. Waters's research can be found by visiting the GRM cancer center at Purdue or googling The Old Grey Muzzle Tour.