Why the age of retirement is generally accepted to be 65?

In 1889, Germany became the first country to adopt a national pension program under the leadership of its first Chancellor,  Otto von Bismarck — the Iron Chancellor. The German system initially was intended for those 70 and older, but the starting age was later lowered to 65, according to historians. The average life expectancy in Germany – at that time – was about 62 and Bismarck figured out that the program would be good politics and doesn’t cost very much.