If you look at the AAHA client handout mentioned above they clearly admit that young dogs don’t need to fast as long as older ones before surgery.
The reason is that older pets have a slower metabolism and it takes them longer to digest food.
Besides their energy reserves are greater than those of younger animals.
Now most dogs undergoing neutering or spaying are young and healthy with a good metabolism.
A small dog should not be made too fast from 6 pm if hes going to have surgery at noon the next day.
According to the ASPCA puppies four months of age or younger should be given half their usual breakfast on the day of their surgery while dogs five months or older should receive a quarter of their usual meal.
As far as the ASPCA is concerned there are no restrictions on water and this makes sense especially when it comes to young pups who are at a greater risk for dehydration.