I don't shop anywhere that specifically caters to the over-40 crowd (although I'm 41), but I have made a real effort not to shop at Forever 21 or the juniors' section at major retailers. I am trying to create a more sophisticated style, and it seems like almost everything at stores geared toward twentysomethings has a particular "feel" to it that makes the item read young. Sometimes just the fabric choice (often thin and clingy) can be a problem.
I was recently comparing two white denim skirts, one from Lucky and one from American Eagle Outfitters, and the one from AE felt younger than the one from Lucky because the AE one had more zippers, plus the zippers were more prominent due to their contrasting color. I think the AE one might have been an inch shorter also. So even small things can make a big difference.
I have been shopping mostly at Nordstrom, Banana Republic (their clothes are pretty conservative and so work for lots of ages), Club Monaco, Ann Taylor, and Anthropologie. Ann Taylor might be the best of these as far as resources for over-forties.
MsMary may not like J. Jill, but I am heartbroken that I can no longer shop there (I am sized out). Some of the clothes were frumpy, but other items were just classically simple and comfortable. And the quality was absolutely amazing. I also am sad that I cannot shop at Eddie Bauer, which may be another good resource for over-forties, seeing as the stuff is pretty basic but high quality.