The age is less important than the weight of your rabbit as a smaller creature will be weaker and the risk of the anesthesia will be greater. For a male, it can be done quite soon. Most vets say around 4 months old. You can do it sooner if your rabbit has grown fast and well but I wouldn't risk it without a good reason. Spaying being a lot more invasive, most specialists prefer to do it as soon as the female is grown (when she has the maximum chances of waking up from the anesthesia but is young enough that there will be no tumors and the vet will be able to remove only the ovaries which makes for an easier and faster operation than if you have to take a damaged uterus with them... it's also the best for a speedy recovery). So, it's around 7 months old for a dwarf rabbit and closer to a year old for a giant. Of course, it depends on the condition of the rabbit and of the vet - some vets are not comfortable to do these operations before the rabbit is a certain age while others don't care as much. But as a rule of thumb, I'd say 4 months for a neuter and 7 months for a spay. That's what I always did, knowing I've owned Holland lops and Nethies. It always went swimmingly.