I am almost there (38) and don't feel at all out of the loop yet! I have been feeling something of a change coming on, though, because I have less inclination to frequent the 'fast fashion' stores, and have recently developed a rather potent aversion to paying over £30 a pop for a piece of shoddily stitched polyester tat. I mean, let's face it, if we abandon the majority of these youth focused stores, what are we really losing? Not much!.
I tend to buy less and pay a bit more these days, seeking clothes which are still current, yet more likely to suit my own personal style and bodyshape.
I don't think that there is anything to steer clear of exactly (I loathe age related fashion rules) except maybe hotpants and cropped bra tops, but perhaps as we grow older it is better to view the road ahead as a new challenge, an exciting new adventure with fashion rather than harboring a sense of having given up (or having to restrict our tastes). Please don't fall for that rubbish!
Most stores cater to the younger demographic because younger women are more likely to fall for the hype (polyester catwalk ripoffs, etc, and stuff nobody in their right minds would consider, especially quality-wise). This doesn't mean that fashion is no longer relevant to the over 35's, but more a case of the over 35's aren't so easy to freaking control (market to).
There's no need to resign yourself to rifling through the frilly isles of per una or any other stereotypically 'middle aged' store, in fact there's no real reason to leave behind any of the stores you may have shopped at previously - just take more notice of your own individual tastes, aspirations, bodyshape and style, and follow your intuition. I still frequent some high street stores, if something catches my eye, but these items will tend to have some sort of 'ageless' appeal, or at least fit in with my personal style/personality. I only avoid Topshop because the clothing depresses me (I love the shoes!), for most of the pieces seem to be designed to destroy the natural waistline, and the quality is pretty appalling.
I love more middle range designer stuff via NAP: brands like APC, Vanessa Bruno (when I am feeling flush!), JCrew, Joules, Zara (my fave currently!), Boutique by Jaeger, occasionally Whistles, Massimo Dutti, Aubin&Wills, and even Jack Wills for those lovely soft cotton shirts, which really suit my figure.
The trick is to match your body type and to seek clothes which will inject a truckload of confidence into you.
We live in an alarmingly youth obsessed culture, but try to remember that this is flaw rather than a given. Young people are fabulous targets for capitalism, they love to reel them in so young because youth is so vulnerable and an easy target for companies which make a fortune selling crap to coddle our insecurities. This was most obvious to me recently when I read an anti-aging cream review site where almost all of the reviewers were under 21 !!!
Don't fall under the spell of this media driven quagmire of ageism in fashion. Style and confidence have no age. Embrace where you are now, who you have become, and enjoy the quest to fill up an entire new wardrobe!