There is no right or wrong answer for this question. I personally spay my older retired females. But I have always spayed them late in life, like around 5-6 years old.
However, there is a recent study, which has been performed in 2009 on Rottweilers. The study included 119 females which have lived over 13 years of age. It is considered as a quite an old age for this breed. The average lifespan of this breed is about 9 years old.
So, it was concluded that out of the group of 13 years and older dogs, females who have retained their ovaries till the age of 6 years or older, were a big part of this group. According to this study it was “found that female Rottweilers that kept their ovaries for at least six years were four times more likely to reach exceptional longevity compared to females who had the shortest lifetime ovary exposure.”
There are also studies which prove differently, stating that fixed dogs live longer than the ones who have not been fixed.
But in any case, you should consider it carefully. The age to do the procedure, my personal opinion, it should be done later in life. And, of course, the vet. Anesthetic , as we know, is something to take very seriously. And spay or neuter is an elective procedure, which can wait till the dog is a little older.
So this study is something to think about. There are also studies which state that spaying early will prevent mammal cancer. It is also true, However, the spay is a total hormonal shift in a female dog , and I personally prefer to spay them, like I have mentioned, later. That seems to be working well.
Also, AKC allows spayed and neutered dogs to compete in pretty much all companion/ performance events, but not in Conformation.