The Collaborative International Dictionary of English v.0.48:

fifties \fifties\ n.
   1. the decade from 1950 to 1959.
      [WordNet 1.5]

   2. the time of life between 50 and 60.

   Syn: mid-fifties.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Fifty \Fif"ty\, n.; pl. Fifties.
   1. The sum of five tens; fifty units or objects.
      [1913 Webster]

   2. A symbol representing fifty units, as 50, or l.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fifties
    n 1: the decade from 1950 to 1959 [syn: fifties, 1950s]
    2: the time of life between 50 and 60 [syn: fifties, mid-
       fifties]