Possible negative impacts of spaying / neutering at an early age:

• Extra growth if neutered before growth plates close
• Increased risk of orthopedic disease
• Increased noise phobias and undesired sexual behaviors in dogs neutered before 5 1/2 months old
• Infectious diseases were found to be more common in dogs neutered before 6 months in age
• Increased risk of bone cancer in dogs neutered before 1 year in age
• Some studies also show that agression issues can be tied with early spaying / neutering

It is important to note that there are many benefits to spaying / neutering (besides the obvious), such as decreased risk of cancers of the testicle, prostate and area around the anus. As well as decreased risk of mammary cancer (which is to blame for half of all tumors in female dogs). Neutering before a dog's first cycle reduces the risk of mammary cancer to 0.05%.