Our Agility club has twice hosted Chris Zinc in Australia and her lectures always related to the ongoing affects of early desexing. My wife runs a rehabilitation clinic and we found that by far and away the highest number of CL 's we see are in dogs desexed before they were 6 months old.
If you are going to run your dog in any activity that requires high speed turns like agility, fly ball or frisby, then do not desex until growth plates are fully closed. As a rule of thumb, after the bitches first season and with males 8 to 9 months with toys and minis, 12 to 18 months for border collie sized dogs and 18 to 24 months for bigger dogs.
Border River Pet Resort