http://www.yorkietalk.com/forums/gen...n-if-ever.html
Here is a link to this topic, where-in you can find further links to studies and articles.
Growth plates can close anywhere from 7 mths on up for the toy breeds. There is no guess work. Take a lateral Xray view of the tibia, as the tibial plate is the last growth plate to close.
There is much research in recent years that challenges the assumptions of spaying/neutering at 6mths old. It is all there in the articles linked; as well there are at least 9 studies currently on-going on this topic with publication dates out in he next year or two.
I share what I take right now believe to be the best practices on this question. For females allow maturation to take place, which usually means one heat for the toy breeds, and more heats for the large breeds. For males, there is no clear benefit health wise for neutering at any age, for a normal male. A case is justified if the male does not have two descended testicles.
You as the guardian and health care advocate for your dog, need to understand the ramifications, and to weigh other considerations in your decision; a very very important one, is to not breed and make sure breeding can't happen if you elect to keep your dog(s) intact. That means you need to educate yourself on how best to do this.
One day I would hope general vets will keep up on the current research and begin to advise the clients accordingly.