I would hope it depends on two things before the age of the dog. Number 1 - first and foremost would hopefully be the weight of the puppy. Then it would depend on whether the testicles have 
both descended.  I had my Little Mac neutered when he was 3 days shy of 6 months and he weighed 5 pounds, 1 ounce. 
I'm not a breeder nor a vet - that's just my 2 cents worth. 
