The first thing I thought of when reading this is the 10000 hour rule. These people didn't come out of nowhere. Most of them appear to have been honing their craft for a while prior to becoming famous/wealthy (or at least getting practice in a similar field).
I was going to write something similar but I don't think it's most. Of the 7, only the Nobel Laureate, Ray Kroc and the runner seem to have been working on their "art" for lack of a better term for years beforehand.
Bruckner and Midgley seem to have at least been in related fields though the difference between being a lecturer/teacher and an author/composer is more vast than I think we might think.
Chandler and DuBuffet seem to be the most likely to be deemed prodigies. Chandler especially having been in a completely different field could be considered a prodigy.
Malcolm Gladwell's article on late bloomers is also relevant http://www.newyorker.com/reporting/2008/10/20/081020fa_fact_...