I feel like any general opinions I have on what's aging or too young will just rehash what other people have said and are the type of thing where there's always going to be someone who pulls them off at any age, anyway, so I'll just talk about my own experiences:
-90% of the animal-print stuff out there feels too aging when I try it. I think it's because, with my intense coloring and curviness, it's pretty easy for it to read like I'm trying very hard to look va-va-voom. Which kind of = aging for me. But, obviously, many other people wear lots of animal print in a way that doesn't look too old for them at all.
-I used to wear variations on the kind of outfit with basic jewel-tone tops or twinsets and black pants that Columbine described up thread a lot, and they always made me feel a little like I was dressing too old. I think it's because I was choosing them entirely because they were easy, practical, appropriate and flattering, and not because I particularly liked anything about the look. So I was a 20-something with very few responsibilities in life dressing according to a thought process that would make way more sense for a busy 40-something with a demanding job, kids, and a house to maintain. Also, I'm not sure I wasn't choosing things with the best quality or fit ever, which probably didn't help.
-Some of my beloved casual knit dresses are starting to look too young on me, especially summer ones. I think it's the combination of short length, low-cut neckline, and a kind of shapeless, unstructured cut through the body making them feel a little juniors-ish.
-I wear a lot of outfits with tunics/short dresses, black leggings, and flats on weekends in the spring and fall. I feel like they're probably too youthful (because of the short dresses and the fact that none of the pieces are necessarily great quality) and too aging (because it's a trend that's passed and it's an outfit that's so obviously chosen because it's comfortable) at the same time, but I keep wearing them anyway because they're so useful and I just like them. I'm kind of trying to figure out how to tweak the look, though, so it's not pressing those buttons as much for me.