usually vets that work with shelters will do whats called a "pediatric" spay/neuter at anything from 8 weeks on.
however its generally reccomended 6-8 months for most breeds and later if dealing with giant breeds (like danes) in order to make sure they grow correctly. most breeds are done growing in the 6-8 month range so its safe, but giant breeds take longer to mature.
some vets have an "age" restriction...
other vets have a weight restriction
and others have no restrictions.
for the average pet home/owner i usually suggest the 6-8 month mark
Ok, I will make the appointment as close to 6 mos as I can. I figure they are going to be medium sized dogs and so they should be almost grown by then. But if I see any leg hiking, well I may have to take em in a little early.
It's really a personal decision, and up to your vet. As others have said, some won't neuter until 6 months of age, while others will neuter as early as 3-4 months. I've generally had my pets altered around the 6 month mark, except for my rescue animals, some of which were done much earlier, before they came to me. The longest I've waited is 8 mos. for my great dane.