This has been thoroughly researched by Chris Zinc of John Hopkins University in Baltimore. She established that dogs that are desexed before their growth plates are matured will have issues later in life including a much higher chance of cruciate ligament ruptures.
She recommends not desexing small dogs before 9 months, medium sized dogs before they are 12 months and large breed dogs before they are 18 months old.
I would rather go with proven scientific facts than vets opinion.
As an aside, I wonder if anybody has ever bothered to challenge the shelters and foster groups on this. It seems to me that they are setting dogs and owners up for huge medical bills, and the pain and discomfort of a major op with their insistance of an immediate, early age desexing.
Border River Pet Resort