[FalcorTheDog]

Following up, I plugged in the data from here into Excel: http://www.multpl.com/s-p-500-historical-prices/table/by-yea...

Some observations:

  -there are 122 24-year windows in the dataset
  -not a single 24-year period had a negative return
  -the lowest average return for a 24-year window was 0.15% in the window from 1874-1897
  -there have been only two 24-year windows since 1876 with average annualized returns less than 2%
  -every window since 1912 has been at least 3%
  -every window since 1968 has been at least 7.1% (the author's assumption)
  -every window since 1971 has been at least 8%

Note that this doesn't take inflation into account, but also does not factor in the returns from reinvested dividends. Those effects would probably approximately cancel each other out, but hard to say.