plural of man (n.). To separate the men from the boys in a figurative sense is from 1943; earliest uses tend to credit it to U.S. aviators in World War II.
One of the most expressive G.I. terms to come out of the late strife was "that's where they separate the men from the boys" -- so stated by American aviators leaning from their cockpits to observe a beach-landing under fire on some Pacific island far below. ["Arts Magazine," 1947]