Why is it recommended to neuter at 6 months and not before?

 I adopted a 4.5-month male pitbull/shepherd mix puppy. As of yesterday 8/11, he is 35 lbs. Part of the adoption process is to get him neutered and receive his rabies vaccine. During an appointment with my senior dog I asked my vet what age she recommends a male pup get neutered, she said for small breeds at 6 months or before and larger breeds at 1 yrs of age, this depending on the dog's temperament. I never really got to ask her why 6 months tends to be the ideal time and not before for bigger breeds.

He will be 5 months Aug 22. He doesn't look to be a big dog and he behaves very well at the moment.

Answer

The idea is to allow full growth before neutering (allow the growth platelets to close), so as not to interfere with bone growth. IMO, your vet is spot on with the MINIMUM time frame and I waited even longer with my dogs (both the males and the females)

You are lucky since, in my state, the vets suggest 4 months of age. It is a question of what the state veterinary board tells them to say since they are licensed and have to tow the mark to avoid "complications" if you know what I mean.

If I were you, I wouldn't hesitate to ask your vet questions, about this and any other procedure.

Also, if you remove the ability to produce testosterone too early (the 'brave' hormone) your dog could be quite anxious.