In my limited days of keeping black tree monitors, I've only lost two females, both while gravid, from a respiratory infection. So I have not had any animals die from "old age". Both females I had were rather young(judging by their sizes). So I have no idea... I'll call you in a decade or so...hahahah lmao..
As for the best tree monitor to keep, that all depends on many different things. While I believe that the general care for each species is almost the same, there are other aspects to consider.. In my experience, I think that beccari and macraei attain the largest sizes(from what I have seen), so they would command a larger sized enclosure than that of prasinus or boehmei. Price is another factor that plays a strong role in determining which species to get. Availability is also important, as beccari is probably the most available of the complex.
But just because one animal is more available, or less expensive, doesn't mean that a lesser amount of money should be put into their upkeep. Tree monitors, as well as other indonesian monitors, such as the indicus/doreanus group, are very needy captives. I think that people should gain some experience with some of the cb australian species before diving into a wc tree monitor, for in most cases, they are just setting themselves up for disaster...
Tree monitors demand a lot of your care, money, and time in order to succeed in captivity. It is all to often that people skimp on one or more of these aspects, and the animal/s perish..
So basically, the decision's up to you; which color do you prefer?hahaha... but seriously, make sure you do your homework and gain some varanid experience before attemting a wc tree monitor... that would be my advice, although other people may disagree..
They are extremely frail, and minor mistakes can cost them their lives... perhaps one day, cb animals will be much stronger and resilient, as many of the cb australian species are today... however, I would still think they'd require more attention than the oz species..basically due to their humidity/moisture needs.. which many people neglect...
Well, that pretty much sums up my thoughts, it's all up to you, and your pockets/capabilities... I wish you the best of luck...Take care,
bob

The Odatriad