I don’t know that there is any specific top age for a rabbit to get spayed or neutered. The rabbit’s overall health would be one of the determining factors. For an older rabbit (4 or 5 or older), in addition to a general checkup, blood tests should be performed to check the function of the liver and other organs. A rabbit that is 3 years old would not be considered too old. You do need to have a good, rabbit savvy vet that is well versed in rabbit surgery, of course.
Here is a section from an article from the HRS dealing with this subject:
When is a rabbit too old to be spayed or neutered?
Veterinarians will have their own opinions on this, but in general, after a rabbit is 6 years old, anesthetics and surgery become more risky, but that doesn’t mean it can’t be done. Simply consult your veterinarian regarding your rabbit’s health and circumstances, and opt for pre-surgical blood work.
It is always a good idea, in a rabbit over 2 years of age, to have a very thorough health check done, including full blood work. This may be more expensive than the surgery, but it will help detect any condition that could make the surgery more risky. This is especially important if anesthetics other than isofluorene are used.
http://rabbit.org/faq-spaying-and-neutering/