He explained to his crew that: after death, he is no longer your enemy, and the crew hand stitched a Japanese flag. troops managed a counterattack later that day they found the lieutenant's body along with those of about a hundred German soldiers.
However, on this site, the history of phrases, their origins and the influences on their development is considered etymology--or at any rate, numerous questions about the origins of phrases are tagged "etymology", and answered on that basis.
Why should this question be treated any differently than those?
To me, the question reads as a "What is the origin of this quotation/proverb/saying? Such questions—for example, Origin/first known use of the phrase ' I've got some good news and some bad news'—generally encounter little opposition on grounds that they aren't on topic at EL&U, and I don't see why this one should be viewed as fundamentally different from them.
The first thing to notice here is that O'Rell puts this rule forward as applying to an ideal difference in age between newlyweds—not a maximum age difference.
And sharing the joy of getting a free bus pass at a similar time, or saving money from combining big birthday parties, is clearly attractive.