Jennifer's point about the importance of testosterone in making sure the growth plates close properly is crucial. When the testosterone source is removed, the growth plates stay open longer, giving the dog a weedy appearance and predisposing the elongated long bones to osteosarcoma.
Performance vet Chris Zink has a good web page on the risks of early spay/neuter:
http://www.caninesports.com/SpayNeuter.html
Regarding the age at which to spay, I like to let pet bitches go through one heat cycle before I spay them. Last I checked, there were no formal studies on this, but anecdotal evidence suggests permitting one season might reduce the risk of incontinence and leaking (a risk of spay at any age), perhaps because of the maturation of the sex organs and estrogen receptors.
Delaying spay past the first season comes with drawbacks, notably an increase in the risk of mammary cancer. Spaying before the first heat cycle effectively eliminates the risk, waiting a season raises it by something like 10 percent, and by the time the third heat arrives any benefit of spaying in terms of mammary cancer is negligible.
Of course, the assumption is that the bitch in question would be responsibly housed or kenneled during estrus and would not be at risk of an unplanned breeding.
Denise Flaim
Publisher
Modern Molosser
www.ModernMolosser.com