At what point should you stop dyeing your hair? At what juncture does the idea of you being a natural brunette (in relation to the rest of your appearance, that is) feel a little... improbable?
It's something I have been thinking about a lot lately, as I approach 60 and, after nearly a quarter of a century of having my hair professionally dyed brown, I have come to the decision it is time to stop. Time to admit to the world that I am not, in fact, a natural brunette any more. I am almost completely grey.
And so it is that I am sitting in my favourite hair salon, finally ready to take the plunge. To go out proud and grey, that is, which means getting rid of all the brown and dyeing it to match my grey — no, hang on, let's be clear about this — white roots.
If this doesn't seem such a very big deal to you, then perhaps you are a blonde and can get away with leaving them undone for a while.
For dark brunettes like me, where the contrast is high, it is a different story. In the olden days, I could just about manage visiting the salon only every two or three months. Now that it takes only ten measly days for the roots to grow back in — less for my wretched "sideburns", and don't even talk about my nutty professor eyebrows — it feels almost not worth going home in between.
Am I an anomaly clinging on to the brown? Maybe.
Most of my contemporaries went pre-emptively blonde in their 30s and have largely forgotten what colour they were to begin with. According to Mintel, only 6 per cent of over-55s who colour their hair in Britain go for brown or darker.
The full-on colouring with balayage highlights and so on, which I do every three months, costs alot. I know, but what is the alternative?
Like my heroine, the late, great Nora Ephron, once said, at a certain point in your life you are only ever eight hours away from looking like a bag lady.
She also wisely pointed out that there is no more potent signifier for old age than grey hair.
"There's a reason why 40, 50 and 60 don't look the way they used to," Ephron, herself a brunette, once put it. "And it's not because of feminism or better living through exercise. It's because of hair dye.
"In the Fifties, only 7 per cent of American women dyed their hair; today, there are parts of Manhattan and Los Angeles where there are no grey-haired women at all."
So why do we go grey (and then white)? A lot of it has to do with age and genetics, in that our genes decide our rate of melanin production — melanin being the pigment that colours each hair follicle.
The older you get, generally the less melanin your body produces.
There is also evidence to suggest that hair follicles produce minute amounts of hydrogen peroxide (bleach), which — when it builds up on the hair shaft, the more so as you get older — can lead to losing your hair colour.
On the other hand, the hair trend of the moment surely is just that — grey hair. Thanks, in part, to Emilia Clarke's Game Of Thrones character Daenerys, to Lady Gaga, recently nominated for a Golden Globe for A Star Is Born, and to various Kardashians, 'silver vixen' has become the colour du jour.
It's one thing when you are in your 20s or early 30s and your non-wrinkly skin can pull off a hair gimmick, but, when you are my age, and need all the help you can get on the rejuvenation front, it is quite another.
Tellingly, I have not told my partner I'm doing it, as I know he will talk me out of it. He adores my long brown hair and is always the first to point out my roots growing in.
I think my hair makes him feel he has a girlfriend younger than she actually is. But I must be obdurate about this. After all, you can only kid the world you are the same rich chestnut tone you were in your student days for so long.
What about when I am 80-plus and on my Zimmer frame? How spooky will brunette look then?
Josh Wood, who is also the creative colour director of Redken and counts model Elle Macpherson and Kylie Minogue as clients, is an old friend, as well as my hair colourist.
We met when I was pregnant with my now-21-year-old and writing a piece for Vogue about something or other. Like any brilliant colourist, he is also a brilliant psychotherapist and knows only too well what a big part of my identity my hair is, how it is my story, my armour, my "schtick".
It would be different if it were unremarkable but, though I say it myself, it is not. It could be the Indian genes, it could be all the dairy and animal fat, but I do have unusually long, thick hair.
Wood knows it is all about striking a fine balance. At the same time as not wanting to disappear into the furniture, I do not want to look in any way eccentric. Ugh! That witchy-hair-and-novelty-spectacle-frames look. I hate it.
And, though I am not Lady Gaga, nor am I Christine Lagarde.
But he is on it, as usual — in his blunt Yorkshire, yet super-analytical, way. "Grey being metaphor for 'letting yourself go' is such a pre-conceived, typically British notion," he says. "Yes, there is definitely that shade of 'apologetic' grey, the look that screams 'it's never my right of way', but if done right, it can be quite a powerful and fashionable statement.
"And there's an honesty about it. With all these fake fixes at a woman's disposal, with a solution in every chapter of the ageing catalogue, it feels authentic."
I had wanted to go all-over grey immediately, but he won't let me. Firstly, because it would wreak havoc on my poor hair and, secondly, because, he says, it would not work at my age.
He warns me it is going to take at least three treatments, amping up the silver each time, and that I might have to grit my teeth while we go through the "transition".
The notion, then, that this is going to be less high-maintenance than being a brunette is fading.
If I want that sleek, silvery vixen look as showcased to perfection by actresses Dame Helen Mirren and Diane Keaton or the comedienne Tracey Ullman (who I think looks far better now than she ever did in the Eighties), we are going to have to keep colouring and/or toning it for an indefinite period of time.
To go completely grey/white will probably take two or three years and, all in, cost a few thousand pounds.
That said, when the roots come through, it will not be nearly as noticeable and, because I'll need less colour less frequently, my hair will be in much better condition.
The first step, interestingly, does not involve bleach, as you'd imagine, but vitamin C, or ascorbic acid, which gradually "strips" the colour, rather than bleaches it.
Though quailing slightly when Josh and Danielle, his beautiful sous-chef, start painting on the strange-smelling potion — has life as I know it ended? — I'm ready.
"Greygerous," as Josh calls it, here I come.
So that I can get the full impact, I force myself not to look in the mirror until my hair has been blow-dried. When I first catch my reflection, I do not know what to think.
Yes, it is amazing — how did he do that? — and it is gratifying having all the employees spontaneously gather round my chair to admire it, Adoration Of The Magi-style.
But there is part of me that wants to stop after this first go. Am I sure I am ready for this? There is no getting away from it, no one would describe me as a brunette any more and that makes me feel wobbly.
The second biggest shock is getting up the next morning and taking a first look in the mirror. Briefly, I'd forgotten I'd gone grey and start at my reflection.
But, as the days pass, gradually I get used to it. Surprisingly, my partner loves it. I mean, really loves it, like jumping-up-from-the-sofa-and-having-a-closer-look loves it, but I think this is because it reminds him a little of the blonde streaks I used to have much earlier on in our relationship. In a certain light, before I go in for the second go, I could be an ash blonde.
Ditto my two sons, who were dead set against the idea — children hate it when you do anything different to your appearance, however old they are — but who begrudgingly admit it is "quite cool".
Meanwhile, my girlfriends, many of whom wondered why on earth I would want to do such a thing, are approving. One goes so far as to say it makes me look younger, rather than older.
You may disagree, but there is something to be said, if you have light eyes and olive skin, for a silvery, rather than auburn, hue.
Three weeks on and I'm loving it. Yes, it might mean pasta necklaces all round this Christmas, but I know I'm investing for the future.
Almost every colour in my closet looks better on grey than brunette. In fact, it's like having a brand- new wardrobe.
Another thing: having grey hair makes me look after it better.
For years, I've got away with the messy "bed head" look, but now I am grey, I can, if I am not careful, err perilously into "crazy lady" territory.
There are only two small downsides. One is that gold jewellery doesn't work so well any more — it looks "tinny" on silver hair.
The other is that I exercise less because I'm nervous of getting my hair frizzy from the heat.
Frizzy brown hair is one thing, but frizzy grey hair is a real no-no. I'm going to have to find a way of doing hot yoga without getting my hair wet. Two Speedo bathing caps, perhaps, one on top of the other?
At a lunch, I notice a woman with her back to me with the most beautiful long silver hair half-pulled up in a hair clip. When she turns round, I realise it is my friend Clemmie, a former blonde. God, it looks good.
Meanwhile, as the positive feedback continues and there are no telltale roots (so far, anyway), I find myself becoming judgmental to the point of evangelical as I go about my day.
Having lost my old schtick, I seem to have found a new one and it's far more strident than its predecessor.
Revisiting my initial question, I've decided no one over 60 should be anything other than grey. In other words, if you are of a certain age and notice a woman with unusually long grey hair scrutinising you a bit too closely at the traffic lights or the Waitrose checkout, it may be me.
Overall, I have no regrets, except perhaps, for not doing this sooner. Looking back at selfies from a month ago, I can see that something is off. Josh has a word for it — Lego hair — meaning it wears you, rather than the other way round.
But then, isn't it funny how resistant we are to change and think that what works for us now is going to work for us for ever? So go on. If it's looking a little "Lego", be brave and make that preemptive strike.