I believe 6 months old is best. As Watermelons mentioned, when the testicles drop your rabbit has reached sexual maturity. However, neutering can be a risky procedure. Find a trustworthy vet and good pricing near you by inquiring with the local rabbit shelters and organizations. There is some preparation involved before taking Chipotle to the vet. Check out this site to learn about neutering and what you should ask your vet: