Brain areas associated with self-regulation don't mature until the mid-twenties. And apparently, if you compare older and younger people in a standard set-up for detecting ego-depletion, older people are not affected. WEIRD, indeed.
In 2010, there was a study indicating that ego depletion doesn't affect people who don't expect it to (and that one was run on college students). I wonder if part of the effect might be coming from differences in attitudes about effort between older and younger people.