The Collaborative International Dictionary of English v.0.48:

Space age \Space age\, n.
   the time during which humans engaged in space exploration; --
   generally considered as beginning on October 4, 1957 when the
   Soviet Union placed the first artificial satellite into orbit
   around the earth.
   [PJC] space bar

WordNet (r) 3.0 (2006):

space age
    n 1: the age beginning with the first space travel; from 1957 to
         the present