A couple of months ago a friend asked if I'd write her a job reference. She is bright and witty and sophisticated and for about 20 years has held a succession of powerful jobs in television and newspapers.

Yet this latest position was rather different: she had applied to be a receptionist at a small office building in Mayfair. This struck me as an eccentric career choice for a clever woman on the cusp of 50, but I wrote out the reference and in due course she got the job.

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Last week I had lunch with her and asked how it was going. She told me that for the first time in her life she was entirely happy in her job. At last she had found something that had all the good things about office work and none of the bad ones.

Her routine was soothing. The people were friendly. The work was pleasant. It was also finite, easy to do well, and ended on the dot of 6pm. There were no unmanageable work loads, no ugly competition, no gnawing anxiety that you aren't up to it and that someone else is better.

But best of all, she said, the receptionist's job didn't swamp her mind and her life; instead it left plenty of room for her to think her own thoughts. The only thing that wasn't fantastic was the money, but it was enough and she didn't mind.

After our lunch she took me to see the site of such happiness. I eyed her curvy glass desk and saw through the square-paned windows the bare trees of Green Park. I imagined myself smiling at the hedge fund managers who came in and out: "Hello, it's milder today isn't it?" I could see a certain charm in it.

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Yet what impressed me most about her satisfaction was how it contrasted with the dissatisfaction of almost all my other contemporaries. One word describes how most of us in our late 40s are coping with far more interesting jobs: badly. In varying measures we are susceptible to boredom, fear, exhaustion and frustration. We've all been working for an eternity as it is, but we now realise we'll have to go on working until we are 70 at least and so there is still a long way to go. In all it is not pretty. We feel we ought to leap, but don't how and don't know which way to go.

A relatively sensible article in February's Harvard Business Review attempts to explain why we are getting it all wrong and why my receptionist friend is getting it right.

The rest of us are falling for the most common misapprehension of mid-career crisis - which is to think this is the beginning of the end. Instead the magazine insists that we have more opportunities than we used to. Because we have worked for a few decades, we know what work is like and what we are good at. The trick isn't to hanker after some magical transformation - one minute you are a banker, and then, hey presto, an organic farmer - but to think carefully and practically about what suits you.

When I think about it now I see that, unlike my friend, I'm not ready to get in touch with my inner receptionist. She says the job gives her space to think, whereas I have a horror of unstructured thought. Indeed one of the things that I've worked out over the past few decades is that I need to be wildly busy all the time: in fallow periods my thoughts wander off in all sorts of unwanted directions.

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Still more hearteningly, the HBR reminds us that even though some doors may be closed at 50, in reality there weren't so many open ones at 25. This is a truth that we tend to forget: most people are in a rut from the start, blindly pursuing careers with no idea of what the other options were. When I was in my 20s I didn't feel that I was deciding rationally between hundreds of possibilities, I was simply trying to do what I thought was expected of me, and what my friends were doing. My motivation was to do it better than a tiny handful of people I considered to be my rivals.

Now that I think of it, I have stopped caring about these petty contests. I am doing better than some of my former competitors and worse than others, but either way it doesn't matter any more. This is liberating. I am starting to mind less what other people think of me and that is liberating too.

My friend confirms that she couldn't have become a receptionist in her 20s or 30s. She would have been miserable, unfulfilled and certain she throwing was her life away. At 49 she has nothing more to prove; she has already proved that important jobs are no longer what she wants.

A couple of weeks ago another cheering piece of work was published by scientists at the University of Warwick showing that happiness over a lifetime is U-shaped. It looked at thousands of workers in 80 different countries and found that most people start off happy, and then slide towards misery, reaching a trough at 44. By our early 50s we start to get happy again and by our 60s and 70s happier still.