> The justification is to force people to work until they are too old to do so.

Actually, the justification is to prevent old people from having to work. Retirement didn't really exist until the creation of pension systems in the late 19th century, and the modern social security system was a poverty alleviation measure introduced in the 1930s. Hell, social security was initially resented by older workers because of the cover it gave employers for firing them for being too old.