The Collaborative International Dictionary of English v.0.48:

Middle-aged \Mid"dle-aged`\, a.
   Being about the middle of the ordinary age of man; early in
   the century, it was considered between 30 and 50 years old,
   but by the end of the 19th centruy it was considered as 40 to
   60.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

middle-aged
    adj 1: being roughly between 45 and 65 years old