[yorwba]

tl;dr: ~5.3 years.

I collected all Wikipedia articles on American billionaires who died in 2008 - 2017: http://tools.wmflabs.org/dschwenbot/intersection/index.php?l...

That got me 74 results. I wgot the articles and manually removed the women based on related Wikipedia categories (turns out https://en.wikipedia.org/wiki/Aubrey_McClendon was a man, I would have removed him just based on the name).

Then I grepped the files for '"[0-9]\{4\} births"' to get the year of birth (this string marks the corresponding category). Two men didn't have a known birthdate, so I threw out their articles, too. This left me with the years of birth for 65 American billionaires. I got their years of death analogously.

After that I calculated their ages as the difference (yes, this could be off by +-1 year, but my method is not exactly rigorous anyway). The mean of these values is ~82.6. 29 of them are >= 86. The mean of those is ~91.3, or 5.3 years past 86. Considering the huge error bars, this is well in line with the general population average.