My current vet will not spay or neuter a dog/cat under the age of 4 months. Previous vets also wanted to wait until pups were older, and scheduled a little too late so those couple mis-timings on the spay/early heat and other people not being as conscientious as me and I ended up with two 'oops litters' anyway.
But I wait until they are older to spay and neuter. Their bodies depend on those organs for maturity. The only ones I had that were spayed and/or neutered early were fosters/adoptees from the Humane Society and they spay/neuter waaaay too young IMHO...although I understand the reason they do.