From what I've heard, with rabbits around 2 years of age and older it's a very good idea to get bloodwork done first; the older they get, the higher the odds that the bloodwork will reveal an underlying condition that could make anesthesia more risky. While, as missyscove mentioned, it's a good idea at any age, it's generally something you have to ask about if you want it done.
With senior males, there's little incentive to neuter unless one wants to bond them to another rabbit. The risks are higher and the health benefits are slim to none.
With older females, a spay surgery can add years to their life if it turns out that they already have uterine or ovarian cancer but it hasn't metastasized because the cancer is removed during the spay. If a female is unspayed then - no matter how old she is - I *always* recommend making a vet appointment to discuss the possibility of spaying her, do preliminary bloodwork and (if it's been a while) get her a physical exam. Once the results come back, the potential benefits can be weighed against the risks - for most older females, spaying is the better way to go... but sometimes leaving them intact can be the better way to go.
TLDR: there's no such thing as "too old" in terms of a broad cut-off; older rabbits need to be evaluated on a case-by-case basis to determine if the surgery is worth the risks.
