Fun thread. I'm starting to realize that what goes on and off my age appropriate list is occurring more frequently as I'm gaining more years.
Speaking from personal experience, the question of "is this too young" usually first rears up in your late thirties/ early forties, especially if you have pre-teen/early teen children; suddenly the fashion trends embraced by the 20-something crowd seem a bit too short, too girly, or too body-con to seem appropriate, except for a "date night" with a SO.
The next self-assessment usually hits in the late forties/early fifties because it's often the first time in years when many women finally have the time, funds, and energy to think about fashion and spend time on their personal appearance. To my mind, this is the stage when many women actually become adventurous with their choices and look for ways to incorporate trends to look current but age-appropriate.
To my mind, the sticky aspect of age-appropriate dressing really hits home in the early to mid-sixties. Post-menopausal body changes, greying hair, wrinkling faces, and health issues all start to become part of the equation. Wearing a flattering outfit becomes more important because it usually makes the wearer look more vibrant and youthful. Many trends just don't look that good on an older woman, unless they are executed in a very careful way or are totally consistent with a personal style. And, the speed of the aging process really affects what is appropriate; an above-knee skirt that looks fine at 61 could look dubious at 64, and totally inappropriate at 66. To my mind, this is the stage where having a strong personal style instead of following trends works better for most women who don't live a high-fashion lifestyle. The kicker is, however, that developing a personal style at this stage can be a confusing and daunting task. The guidelines for helping a newbie fashion lover in her late sixties are few and far between, especially if the person is not one of those "super elders" that the fashion world likes to use as role models for "aging gracefully". That's why so many women over 65 end up just wearing elastic-waisted pants and polyester tops.