It depends on the dog. Different breeds develop at different rates, but 4-6 months is a typical age for surgery . Generally speaking, when your dog loses its puppy teeth, it is time to get sprayed/neutered. Most dogs lose their front teeth at a "kindergarten" age, just like people. When they start losing their baby molars, they are around "junior-high" age (We all know what happens to kids and pups at this age ). Puberty is about to set in and it's time to spay/neuter.

For the best advice, ask your vet. He/she will be able to watch your pet's development and let you know when It's time.