MacArthur famously uttered the words, "Japan is a nation of twelve year olds". Why?
The full quote is actually: "Measured by the standards of modern civilization, [Japan] would be like a boy of twelve as compared with [the Anglo-Saxon] development of 45 years,”
In the 1800's and early 1900's, Japan placed great emphasis on Westernizing. They brought American and Western military leaders to Japan to modernize their army. In many respects, Japan had become very modern and "Western" by WWII, but it was still a feudal society with an emperor, as well, with many conservative, traditional customs. Read more about the reasons for these historical developments in Japan.
McCarthur is expressing his view that Japan was still "backwards," but on its way towards becoming, in his view, a "civilized" nation.