once their testicles have developed and, er, dropped, so to speak. usually around 5 months of age. i waited until my bun was approximately 6 months because i wasn't sure of his exact birth date.
Yep, as everyone said, you can get them done as soon as their testicles drop. I had Starsky done at around 12 weeks old. In general younger rabbits cope and recover better with the op and with anaesthetic than old rabbits do.
The 3 boys are dwarflops and we had a good look at them over weekend and it doesnt look like their testes have dropped yet but as soon as they do it will be OFF with them!!!