There are several great articles to read if you want some more information:

UC, Davis early neuter study:https://www.ucdavis.edu/news/spaying-and-neutering

UC, Davis research abstract (technical read) : http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055937

Dr. Chris Zink:  http://www.caninesports.com/uploads/1/5/3/1/15319800/spay_neuter_considerations_2013.pdf

Puppy Culture Age Appropriate Exercise: https://www.puppyculture.com/new-appropriate-exercise.html