There is not a lot of data on the effects of spaying dogs and cats at younger ages than the "traditional" 6 months that has actually been published, at least that I am aware of. There are reports of two or three places studying the effects of early spay and neuter at the present time and I have seen references to data from these studies but am not aware of published data.

I am only aware of one study of long term effects of spaying and neuter dogs at 7 months of age that followed the dogs for their entire lifetime. In this study, there were no problems attributable to spaying or neutering at this age other than a slight increase in the likelihood of estrogen dependent incontinence in female dogs. A lot of vets are spaying pets at 4 months of age pretty routinely now and humane organizations have been spaying animals in their care as early as 8 weeks for several years. Soon, there will be enough of these early spay and neuter patients and they will be old enough to figure out if there are long term effects. At present, no one is reporting any problems as far as I can tell.