"Aged" is more correct, but is rapidly being replaced by "age", to the point that "aged" sounds fussy and overly formal in most circumstances (at least to my American ear). In many cases, removing the word altogether is the best option and still leaves the sentence perfectly comprehensible. Here's how I would rephrase Jektor's examples:
"A total of 133 children aged between 6 and 18 years of age attended."
(OR "A total of 133 children aged between ages 6 and 18 years of age attended.")
"People aged 65 years and over suffer from a relatively high risk."
"44% of Canada's working-age population (those aged 25 to 64) had a college or university degree."
Yes, I kept "aged" in the last example, though it could go either way. To me, "aged" sounds most natural when referring to a range.