12 of our 15 have been either spayed or neutered (Jessie goes in on the 28th since we thought she had been spayed and lo and behold she went into heat last week). None of ours have had an adverse reaction regardless of the age. In fact, we generally have to force them to lay low for the first night...Sox was running around like a little banshee and jumping up on the bed right after we brought her home. Dr. B. told us to limit the amount of water (a few sips the first night) and the amount of food (a bite or 2 if they want it) for the first day and then it will be fine. We have him trained...he doesn't even ask anymore if we want them to stay, because we had an argument about it the first 2 of them and we insisted that they come home because at least we would be there overnight, whereas he wouldn't be in the office. He did recommend that we NOT neuter Butterscotch and Dudley because of their ages (and Dudley's health), so we aren't. Good luck and let us know how it goes.