My vet office won't do it until the pet is 6 months unless the animal is very well developed &/or having health or behavioral problems related to spaying/neutering. It is safe for younger dogs as long as they are healthy but because of developmental issues it is best to have it done around 6 months.

Most shelters do it when the pup is younger just to ensure it is spayed/neutered as it prevents more pups from entering the shelters.

As far as waiting until the first heat is actually more unhealthy for the dog as it has a greater chance of getting cancers in the mamory glands, uterus, overies and other reproductive parts, as well as pyometria (infected uterus which can be deadly & only cured by an emergency spay).