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.