If the crisis seeps in, if the start is silent, you need a jolt to realise it. Having my daughter was my jolt.
Our second child, she arrived late (five years after our son), a quarter-year before I turned 44. My husband and I knew we were very lucky. One day I was in the kitchen. I looked at my daughter and thought: You are amazing. And then I thought: By the time you're 18, I will be over 60.
I thought: When you're 18, I'll just about have the strength to push you out of the door and into your adult life before I have to check into an old people's home.
Then I thought: If I'm tired now, it's nothing compared with how knackered I'm going to be dealing with two teens in my late 50s. Plus, I still have things I need to do!
I looked at my daughter and I suddenly knew that I had less time to go than I had already lived. That the time I had was a limited resource, that life was an astonishing gift and both were diminishing.
When we're young, we like to be scared by death, because it seems so remote. But in your middle years, it starts moving closer to you.
I'd wake at the wrong time and start mentally choosing the bag I'd take when I left, packing it, imagining how long I'd last on my savings (not long). I'd be rediscovering the old me. I'd be living life gloriously.
I talked to my husband. I said: "Would you mind if I staged a mid-life drama and wandered around for a couple of months?" He said: "Not a bother. As long as you take the kids."
So I talked to other people. I tried to separate the personal from the universal. There's an element of middle age that is the same for anyone who thinks about it. Your potency and potential. Your thrusting, optimistic, silly dreams, have been forced to disappear.
More prosaically, you can't progress in your job: your bosses are looking to people in their 20s and 30s because younger workers don't cost so much or - and this is the punch in the gut - they're better at the job than you are.
Maybe your marriage turns strange. You don't understand each other any more. In short, you wake one day and everything is wrong. 'It's all a mistake!" you shout. "I shouldn't be here!
There is something terrifically embarrassing about middle age. We are of the mind to be young or old. There is cachet in both, even dignity. But not in between. Not in the middle.
So gradually I came up with a plan. I would look back quickly, just long enough to investigate my assumptions about adulthood. I would recall my 20s, check in on my 30s.
And then, I would arrive at my late 40s and I would look at that. At this middle decade, between the old age of youth and the youth of old age.
I would think about what I looked like. What my body can do. What marriage means, what happens when it changes over time. How we work now. Money, which leads to jealousy.
Anger and patience, how they grow or die.
And time. The time left.
And this is what I found . . .
It's ok to be in the middle
We are all trying to tell other people something important about ourselves, and that important thing is how well we're doing, according to how we want to be defined.
Status, we love it. We all compare. But the thing is, if you compare yourself with people you've known for some time - school friends, fun mates, old work friends - you will be somewhere in the middle.
In almost every aspect of life, we are somewhere in the middle.
Don't be complacent
It doesn't seem so long ago that every summer contained a wedding. I used to bitch about them: the expense, the travel, the stupid sentiment. Now, I want another wedding.
I talk to someone I care about and she mentions, lightly, that she and her husband don't agree about anything any more and that their marriage is over.
But they can't separate: not just because of the children -though they are a factor - but because they can't afford it.
"I can't pay the rent on my own," she says. "Neither can he. We're stuck."
And then I drop the kids at a party and find myself in the road hugging a mum I know only slightly because she tells me her husband has been having an affair. That he is leaving.
And she is being funny and strong, making jokes about hooking up with old boyfriends, and hopping on to her yoga teacher. But then she describes how confused their twins are about the whole thing and her eyes fill with furious tears.
"The worst thing is," she says, "that I still love him. I still fancy him. But now I have to stop. I'm being forced to give up on something I still believe in."
Between 2002 and 2008, the UK's Office For National Statistics reports, the number of divorced women over 45 went up by a third.
Part of the horror is the cliche. We all made a promise, when we were young, that we would be different. We would never resort to those tired roles.
But here are our friends, shouting along with the soap script, playing their parts as the vindictive husband, the philandering wife. It's so hammy! But it's real.
Also: is divorce catching? What was so bad about the relationships that have ended? They didn't seem so different from ours, from everyone else's. Funny, attractive, loving people in funny, attractive, loving marriages.
Big pants are best
So much of marriage is arrangements, division of labour, checking of details. It's hard to inject romance into a husband-wife exchange when the essence is to establish which one of you was meant to buy loo roll.
I think our marriage muddles along ok. But maybe I'm complacent. The other day my husband shows me an item of clothing he's found in his office, which doubles as our spare room. He dangles it in front of me. It is a white lacy thong.
"Yours," I say. "Or my dad's."
Definitely not mine: I hate no-cheek knickers. We do the only sane thing to do with a thong - which is to put it on your head and make big googly eyes through the leg-holes - and then I sling it in the bin.
I don't feel any paranoia about the thong. I presume it was left behind by one of our friends who stayed the night. It was a bit dusty: I feel more paranoid about how dirty our flat is.
Although I can definitely feel jealous, I've never spent too much time worrying about whether my husband might leave me for someone else (who wears small pants). If he decides to cheat on me, then really, truly, what can I do about it? If he no longer fancies me, or no longer respects me enough not to have sex with someone else, then complicated new knickers are not going to make him change his mind.
Stay fit
After I had my daughter, I did very little exercise. My body changed. My life changed. I found it hard to locate the person I was before I was a parent.
When my daughter was a year old, I decided to take up running. I wanted to be outdoors, I wanted to be alone, I wanted my head to empty and my body to stretch. And when I got home, the serotonin rush was so strong, my head felt as though it was hosting a firework display.
"Look, you can't get away with doing nothing once you're 40," says a friend. "You have to do some type of exercise or it all goes wrong. If you don't exercise, your back stops working. It starts to go all rigid. You seize up. All of you. Your mind, too."
In the end, fitness - health - is about that energy, I think. About increasing it, lengthening its span, catching it from other people and new experiences, drawing it into your heart from a walk on a cold day.
Let go of your youthful looks
When I tell someone my age and they say, "Oh, you don't look it," I say, "Thank you, that's nice" (it is). But it isn't the point. I am my age. This is what my age looks like on me. What is the point, and is more interesting, is what's happening to me, and to you.
The old me is disappearing. Another me is emerging, pushing through. As we age, we shed our skin and a new person reveals itself to the world.
Something happens to your body in midlife. It happens too slowly for you to notice and too quickly for you to do anything about it. You start to smell different: stronger, more pungent. Your bum starts sagging, your stomach expands, your hair changes texture and colour and begins to recede or fall out.
Your thighs increase in size and change consistency, and so do your arms, which become like lovely hams, and this all happens, pfff! Hello, new you.
Your body becomes less efficient. You can't shake off a hangover. At social events, in darkened rooms, with wine and no dancing, you observe some of your friends are fading at 10 o'clock; drooping eyes, lolling heads. A cab, which used to be a place for arguing or snogging, has become a bed on wheels.
. . . But there's always botox
I bump into a friend I haven't seen in a long time. She tells me she's had Botox in her forehead and filler in the lines around her mouth. I'm so surprised - shocked, really - that I gape at her. She looks normal, like a middle-aged woman.
She says: "I want my looks not to be an issue. I don't want to look younger, I just don't want my looks to be in the equation at all. I don't want to look like I'm 25, or even 35. I want to hover at 40 for the next 20 years. I'm liberal, I'm a feminist, I don't think women should have to do these things.
"But I'm a pragmatist, too, and people do look at your face and they make judgments."
I talk to another friend, a journalist who has written about beauty treatments.
She says: "You don't have to do anything. But this stuff is out there, and you should know about it."
I feel as though everyone is in a club except me. Perhaps, I could join. I read an article that says there is evidence having Botox changes your mood.
If you stop yourself frowning, you feel less frowny.
"Getting Botox is a sackable offence," says my husband. "If you got Botox, I'd divorce you."
I say: "You wouldn't notice."
But I don't get it done.
Being invisible has an upside.
Really, I can't bring myself to care if men fancy me or not. In fact, if they're not my husband, I'd rather they didn't. Like most women my age, I've had years of being shouted at, leered over, told where I rank in a sexual preference table. It's a running commentary. It affects your confidence, it wastes your time.
When you're older, it doesn't happen as much and, honestly, this is a relief.
In many situations, I don't want to be noticed at all. I like looking at other people, watching what they're up to. I enjoy eating in cafes by myself. Checking out art galleries, going for a run, watching films . . . often better alone.
Try not to judge others
When you meet someone you haven't seen for 30 years, what always strikes you is how different they are from how you remember. He's got so fat! And bald! She's gone for a new hair colour and she has that flushed look all across her cheeks and nose . . . I don't know if I'd have recognised them if they hadn't said hello.
And then you stop to talk and you realise the bits that have changed are not the vital ones.
Because the way they move is still the same and the voice and the humour and that endearing habit of laughing in the middle, rather than at the end, of a joke, as though the anticipation is more enjoyable than the climax. The fundamentals remain.
You walk away, and you say to yourself: "It's amazing. They haven't changed a bit."
Let go of the past
The only way to feel lighter and better about going into the second half of your life is to forget most of what you've done before. So shed the clothes you never wear. Call the people you've always liked, meet up with them but don't talk about the past.
At some point, you have to say, "This is as good as I can be, as good as I can be right now."
You don't have to be happy about it, but I would recommend that you are. If you can understand your restrictions and accept them, you're every age and no age at all. You can't change your spouse unless they want to change themselves.
But we can make tiny changes. We can take on the responsibilities we shirked when we were young. We can decide to reclaim our body from outsiders: to ignore what we're meant to look like. Dress it however you bloody well like.
Midlife is different from youth and it's different from elderly. But all of life is living with death flickering and glittering somewhere to the side, around our blind spot, where we can't quite see. Life feels like a chess game. I see the pieces that remain, I mourn the ones I squandered.
Still. All those pieces! On the board! Waiting for me to move them. I think about that, and I feel . . . better.
Because, yes, this is the game. This is our life. Let's play, with whatever we've got left.
* Adapted from Out Of Time, by Miranda Sawyer
