I use a Holistic vet and he recommends that you wait till he is at least a year old so that he can mature. BUT if the dog is starting to get out of hand behaviorally, like stressfull marking(all they think about is marking when in a new area) obsessive mounting, wondering off, etc. it is best to just do it right when that starts up. His personality won't change but he will over all be a well rounded dog. Again, he might still occasionally mount and mark but he probably won't stress out about it. If you don't plan on showing, just get him fixed and same with your little dogs, if they are for showing, get them fixed.
I had my pit pup on a little play date with a 2 year old Rottie today and my pup humped him several times and he is only around 3.5months. His is probably just a dominance thing but still...