Though I retired from breeding a few years ago, I would be glad to share my perspective
While the breed I worked with (on a very select and limited basis) is a bit different, one reference I can give is that membership in a CFA club I used to belong to was that one would not place a kitten prior to 10 weeks...my own preference would be 12 weeks, based on my experience with how kittens grow and mature. Kittens are still interacting with their siblings and their mom, still learning some begining manners, and I prefered knowing that all shots had been given for their first year of life.
My breed, the American Curl, I did not place until after 16 weeks of age due to the fact the shape and tightness of ear curl could change up until that point...even in an obviously pet kitten, I would not sell prior to this age, and in my last years as a breed, sold my pets already altered - I preferred doing their recovery care and knowing there would be no "ooopses".
Fwiw, my understanding is that puppies are a totally different matter...they need to be placed by 6 to 8 weeks for bonding purposes....but someone more familiar with dogs may say that's bunk