This is an area of controversy. There is a statistical association between some issues found later in life, but a causal relation has yet to be proven. Essentially increases in cruciate rupture, hip dysplasia and bone cancer have been reported. It is easy to associate the weight gain commonly seen in spayed females with the cruciate and hip issues.
This is more of an issue to be considered in large breed dogs and working dogs. The increase in incidence of these is WAY less than the increase in incidence of breast cancer associated with delayed spaying, especially in small dogs and house pets.
As in most things in life, risk vs reward.
Doug, DVM