Well, it doesn't matter how old the bunny is; what really matters is how healthy the bunny is. To ensure your bunny is healthy enough to undergo a surgery like this, ahead of time, have a annual exam done (by a *rabbit savvy vet* ) in addition to pre-surgical blood. Though it will cost you extra and some offices may not require blood work, it is very wise to do so. Also, it is key that your bunny is sent home on pain meds. Some clinics say the buns don't need to be sent home on anything; can you imagine a surgery without pain meds after?!
Age wise, to give you an idea, my bunny Ripley was neutered around 1 1/2 and Taffy was spayed at 7 years of age. Both came through wonderfully and healed up really well with no complications. Also, both were sent home on 5-7 days Metacam. I was confident in my vet's abilities and know he's very rabbit savvy and a good surgeon and the technicians in the office are very good.