Well, you could repeat it for all the 24-year periods from 1917 through now. I wouldn't expect very different results, though accounting for college deferments will be fun.
with only 4 observations, "common sense" and other forms of first-principles reasoning are much more important than the 4 data points. also one should consider data from other countries and other historical eras.
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%
My point is that having 100 years of investment data is much more like having 4 data points than it is like having 75 or 75 *12 (if you cut the years into months). Even though you literally have 75 data points, they are pretty close to a copy-paste of each other; not statistically independent.