While wandering around the Internet I discovered an archived story from National Public Radio in the US called "Does Age Quash Our Spirit of Adventure?" The reporter referenced psychology professor Dean Keith Simonton of UC Davis, citing his idea that those who are "eminent"—defined as those who had been quite successful early in life—tended to be locked into the patterns of early life, while those who are, ahem, "late bloomers" tend to remain more open to new ideas in middle age.
This sounded like pop psychology to me, but interesting enough to send me scurrying around the Internet looking for more. I couldn't find any other mention of Professor Simonton's theory of eminence and a decline in creativity, but I did go on to read several articles on creativity and novelty-seeking in middle age.
When I was growing up, "middle aged" was a synonym for "boring." Looking ahead across the gulf of years it appeared to me to be a time of life inhabited by people content with a dull routine, with little interest in the new.
Having reached the respectable middle age of fifty-two, I'm happy to report the reality I've experienced is quite a bit different from that.