What a horrid expression. Even before I knew its meaning the image I had was Tenniel's illustration of Alice rowing the Sheep in "Through the Looking Glass". But that was a sheep wearing clothes. "Mutton dressed as lamb" is far worse. The expression refers to a scurrilous practice of British butchers to trim and display ("dress") a cut of mutton (old and tough) to resemble lamb (young and tender), thus pulling the wool over the consumer's eyes. It's current use describes an ageing woman who is dressed and/or made up as if she was much younger. She's usually thought to be delusional.
We've all seen women who are still dressed from the '80s although they're in their '70s. Those women have just given up and probably aren't reading this. Many women go to the other extreme and try so hard to avoid looking dressed too young that they suck all the fun out of new fashion and shopping for themselves. They are paralyzed they'll make this most horrid of fashion faux-pas. As mentioned, this is the number one question I get whispered in the fitting room. My answer of assurance is to point out that there is nothing inherently "young" about a beautiful color, a new and sophisticated cut, a perfect fit and/or a darn nice figure. Usually a woman knows if something looks good; it's the reassurance that's needed.
However there are some youngish details to avoid. As with all things Fashion there are Exceptions.
> Short puffed sleeves. A long puffed sleeve can be drapey and sophisticated, but a short one is little girl.
> Peter pan collars. They're back and— yes— they're very ironic on a sheer black Stella McCartney. But peter pan collars per se (especially on a puffed sleeve blouse in a ditsy print) are as ironic as lead.
> Ditsy prints. Ditsy prints are those twee little flower designs that used to be on your granny dress. Need I say more?
> Sleeveless. A lot depends on your arms. If you really want to invite envy show off your toned arms. Actually I wish you would; I love to see a woman with toned arms. If they're just "okay" and you feel okay about it, go for it. If you want to try a bit harder, find an armhole that has just a slight drape or extension onto the shoulder. You will look a bit better and probably relax a whole lot more.
> Length. Once again, your legs will tell you. But no one's knees are beautiful no matter your age. There is not one ideal hem length; it depends on the cut of the skirt. What you need to do is add the right leg covering.
Short pencil skirts worn under tunic tops, short flippy A-lines and fluffy party skirts need to be short enough to work the outfit but not higher than the lower third of your thigh. Knee-length (just at the top of the knee) is the standard ideal for daytime or dressed-up pencil and A-line skirts. The midi length is just below where knee meets leg (longer is just old). This is the length of the Mad Men pencil skirt, a softly full pleat or a walk-in-the-country tweed A-line. Today's long length can be a skinny sweater knit, a slight A-line silk or a full-blown ballgown. Your leg will still show as you move so coordinate with the shoe or boot you've chosen.
The rest of your leg needs tights (textured or plain) (color if you like that) for day and sheer or sheen colored hose (black or neutral) for evening. I'm so happy that sheer hose are back for day as well (supposedly we have Dutchess Kate to thank for that). Like a great face foundation, the right hose will just make it all look better without screaming cover-up. Modern day hose does have a bit of texture or shimmer. Let's all try it.
> Bows in the hair (think Sally on "Dick Van Dyke") or sashes tied as bows. Knot or obi-tie them instead.
> Overt sexuality (ie too much cleavage). Is your cleavage pleated? Think again.
> Screwy proportions. A short back pencil skirt with dark patterned hose? Check. With sky-high stilettos? No. Try a chunky strapped mid-high heel instead.
Chances are if you're fearful of looking age inappropriate you won't, but don't paint yourself into a "safe" corner. Harper's Bazaar has a great feature, "Fabulous at Every Age" anchored by a celebrity in her 20s, 30s, 40s, 50s, 60s or 70s+. She looks terrific, and the choices the editors select for each age are great, but I laugh because there's not that much difference in many of them. It's about putting together the right proportions for your body, never skimping on getting the perfect fit and wearing it like you own it. Because you do.