Training and proper socialization should start early and continue through the first year of life.

“The rationale for enrolling puppies in such classes is based on research which shows that dogs who don't have adequate contact with other dogs and people during the early months of their first year of age are apt to become more fearful, aggressive, and uncontrollable.”

https://www.psychologytoday.com/us/blog/canine-corner/201905/do-puppy-classes-have-effect-later-dog-behavior