- Recommendations are 12 months for females and 18 months for male dogs.
Neutering male dogs earlier than 18 months negatively affects growth and development.
Male dogs need proper hormones  to develop body structure normally - without it they grow
tall and narrow and never develop the width and head type that they would grow if left intact. 
There also seems to be a higher incidence of bone  cancer in males that are neutered early.