I found this website that talks about neutering and the ages. There has been a lot of talk about people wanting to wait till the dog is older or gone through the first heat. I like the information on this and think it has a lot of good points.
http://www.petresource.com/articlesofinterest/newviewsonneutering.html
http://www.petresource.com/articlesofinterest/newviewsonneutering.html