From WordNet (r) 3.0 (2006) [wn]:
middle-aged
adj 1: being roughly between 45 and 65 years old
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]