Let me start by saying that I abhor the term 'middle age' on the grounds that it amounts to little more than discrimination and is in danger of leading to fashion segregation. What is middle age these days anyway? And, supposing one could define it, would one want to be shuffled off into a group whose tenets supposedly include 'no long hair, no peroxide, no miniskirts and no fishnets'? I paused before I wrote that last word because I do actually think that the wearing of fishnet stockings by anyone over the age of 25 or under the age of 14 should simply be banned.

This is not a matter of age appropriateness (AA); it's a matter of dignity. In my book, there may not be a sharply defined perimeter fencing off the concept of aging, but there are absolutes where 'the dignity index' is concerned. Madonna, a former hero, fell off my dignity index long ago (witness her recent fishnet fiasco, if you dare); conversely, Victoria Beckham skyrocketed up the index the moment she began dressing properly, stopped singing, returned to her natural brunette and got a decent day job.

I know I sound patronizing, but that's because I speak from experience falling, as I do, off my own index quite regularly. My own critics (aside from myself) are two members of the opposite sex aged 15 and 13, respectively, whom I happened to have given birth to. That doesn't cut me any slack in their eyes. Here's a smattering of responses to outfits I have attempted to leave the house in: 'Why are you wearing boy's clothing?'; 'Are you auditioning for an Abba-tribute band?'; 'You know you can't walk in heels, so why are you trying?'; 'Mum. Do not get out of the car dressed like that.'

Most recently I have teetered on the edge of the index over the wearing of shorts, the acceptability of which turns out to be 'big' among, ahem, a certain group of us out there. I own two pairs of shorts, black and grey, from T by Alexander Wang. Purchased at my very favorite place, TheOutnet.com, they are baggy, pocketed and made of sweatshirt fabric. They rest halfway up my thigh and I wear them with leggingsunderneath and a baggy sweater on top. Do I consider this to be dignified and AA? As it happens, I do. Would I leave the house bare-legged in short, fitted shorts? Only if I were living in Goa and never planned to require anyone to consider me credible again. Do I consider short shorts to be wearable in the context of work and play? Yes, but seriously (and here's an edict), if you are 40-plus, have great legs and are intent on wearing shorts to do business in, then said shorts need to be part of a shorts suit as in those sent down the runway by the likes of Jil Sander, Givenchy (though I fear these were too, too short and slinky for the dignity index) and Proenza Schouler. Even then, I would be wearing them with a pair of footless tights and some flats or three-inch heels at most.

I'm still not sure, though, that short suits are the way to go in the office. Maybe in a couple more seasons, when everyone (men) has gotten used to the silhouette in the same way they did with the mini, shorts won't look so, well, short. I know they've been around for a while in fashion terms, but in the evolution of business and AA dressing, they are still amoebic.

Right now, for those of us over the age of 30, shorts should still be considered experimental and fun. Think, then, about buying a pair of the more outrageous shorts from Stella McCartney (Hawaiian-print cotton shorts, £570), Dries Van Noten (black and white desaturated floral print, £255), Carven (alpine-printed silk, £220) or Yves Saint Laurent (poppy-print cr pe shorts, £640). On top, keep it simple, and if you are worried about modesty and dignity, then keep it loose. Try a slightly oversized white shirt by Equipment (I like their silk tuxedo shirt for £255), Gap's dolman-sleeve white shirt (£29.95) or, if you want to really splash out, The Row's white, long cotton shirt (£610) is a great long-term wardrobe addition. The trick is to get the volume just right so that you look neither too 'gym-ready' nor too blousy. T-shirts are fine, but they should be loose, round-necked and in very good cotton. A friend of mine swears by Iro, but I love tees by Gap ('Mason' crew, from £12.95), James Perse (from £65) and Petit Bateau (from £25). Flat shoes, please (a column on this is just around the corner); Bottega Veneta, Lanvin and Balenciaga are all good places to start.

If you want to experiment with the idea of being taken seriously in shorts, a good idea might be to buy a simple, neutral, tailored pair (best from Theory, £165; MHL by Margaret Howell, £155; and YSL, £455). Revert to the T-shirt idea with either a cropped jacket that hits at the waist or a long, boyfriend-type jacket that ends roughly where your shorts end, or perhaps a little lower. Reach into your wardrobe for these. There's no point in buying something new when you are working with an experimental look.

With the weather as it is, you may still be wearing flat boots and leggings with the above. If it's warm, a flat, fun, strappy sandal will offset the severity of the jacket. Try Pierre Hardy's suede and metallic-leather sandals (£540), Sam Edelman's leopard-print, pony-skin sandals (£68) or choose from the chic range of Ancient Greek Sandals (from £95).

Shorts are versatile and they have the capacity to (positively) change your silhouette. The thing to do is to play with the proportions and see what works for you. It's unusual at this stage in the fashion game to come across anything that can effect such a change, so have fun with shorts; don't take them too seriously and, above all, do not wear them for the first time on an occasion where one simply must get it right. If you feel 'wrong' at any time, remember that even the worst mistakes can be rectified by throwing on an oversized cashmere cardigan in a pretense at insouciance.

I do hope that Anna Morel, director of London public-relations agency Bryan Morel, won't mind me singling her out as a woman who, by dressing simply and chicly, can defy all fashion dictates that generally apply to middle-age. When I saw her rushing around at the John Rocha's fashion show in February dressed in one of Rocha's jumpsuits (from £395) or onesies or playsuits, as they are sometimes called I was forced to reconsider my view on the all-in-one. Anna's was black silk de chine, created with the simplicity that Rocha excels at, and she was unadorned (very important), save for the inevitable phone earpiece and ID badge.

Since then, I have bought (but not yet worn) my own onesie, which happens to be in short form and, again, from T by Alexander Wang (£74 from theoutnet.com). Another friend reports success in hers from Michael Kors in brushed silk (£285) and I would also recommend them from Phillip Lim (from £695), Halston Heritage (from £450) and DKNY (£370). Loose fitting, flattering all-in-ones cover a multitude of sins and shortcomings if you find the right fit. Belt them if they feel too blousy and tent-like; buy the next size up if they feel too restrictive. Get the look wrong and you will look either like a sack or, to put it bluntly, as if you are trying too hard. That's a cardinal sin for those of us over 40.

My onesie advice? In shorts form, flats and leggings; in longer form, a heel is fine and practically mandatory if the onesie is in evening silk with wide pants