I was researching this while I was waiting to get Abby. There is actually a new line of thinking that says its best to neuter or spay as early as 8 weeks. The thought process behind this is that an animal "altered" before 6 months of age has almost a 100% chance of remaining prostrate, testicular or ovarian cancer free. Research has shown that younger patients recover faster and have fewer surgical and post-surgical complications than their older counterparts. There is very little to no body fat to contend with, the incision is smaller, surgery time is reduced, recovery time is very short AND dogs will actually grow a little larger due to longer growth periods causing increased bone density. (told ya I researched this! )
I don't know if or when I am going to get Abby spayed. Because of her issues, I am leaving that completely up to her doctors. She needs to be physically strong enough and we will need to be sure her kidneys wont suffer from the anesthesia. One of her doctors still says that it's doubtful she will ever reach the age where we even have to consider it.
But the general consensus of doctors today think a good age for any animal (male or female) to go under the knife is between 4 and 6 months of age.