Here is a link to a free public webinar on the optimal time for spay/neutering sponsored by the Association of Shelter Veterinarians:
Spay/Neuter Webinar
The presenter is Dr. Philip A. Bushby, a professor of veterinary medicine at Mississippi State University and an expert on this topic.
The webinar is
TOMORROW (Thursday, February 22, 2018) at 9 PM eastern time and is free and open to the public. I'm not sure whether they will make a video of the webinar to watch later, so if you're at all interested in this topic, sign up ASAP.