I'm reminded of my trip to IKEA with my 70+ mom. My mom received one of those shocks that kind of upset her - the kind where you think a person is young and they turn around and you see they're old. But the thing is, this *old woman* was 30. Not old so much as adult. And what was interesting was there were tons of people - from 5 to 70 in leggings and my mother didn't bat an eyelash. The lady wasn't wearing leggings like everyone else; she was doing '80s Sarah Jessica Parker - a high school look from days of yore. I think that's what got my mom. My mom remembers that as a high school thing and she was expecting to see the face of a giddy sophomore, not a stern mother of two.
My two takeaways: 1) leggings are standard, like a folk costume and 2) maturity may really be the sticking point with people, not age per se. Like maybe leggings under shorts is best left to the kids...