FOR MOST of my life, I was the youngest person in any group. In Birmingham, back when I grew up, if you were born late in the calendar year, you went to school early. I was four when I started primary school, and 10 when I started secondary school. In my first job in ITV, I was running my department at the age of 26, at least a decade younger than my peers.

But now I’m often the oldest person. I was just talking to my daughter about the labels we attach to generations. She’s either Gen X or a Millennial, or – as she claims – something called an X-ennial, born right on the cusp. She checked (on her phone, of course) and confirmed the dates and the sequence.

This was bad news for me. I knew I was a Boomer, but was horrified to find I’m actually a Boomer One – from the older half of that generation. Not even a Boomer Two. The frozen remnant of my old ITV pension has been paying out for seven years so far.

I just claimed my UK state pension. In America, where I live, I’m on Medicare, the state-run, old-person healthcare scheme. I’m a senior citizen now.

This makes it hard to say exactly where Jack Reacher came from.The obvious answer is from inside my head, which in turn depends on everything I ever saw or heard or read or thought or experienced – but that was all Boomer One stuff, shaped by a specific historical era, now long gone.

So why today does Reacher appeal to readers of all ages? The oldest of my fans I ever met personally was a Scottish lady of 101. The youngest was a boy from Milton Keynes, aged nine. What were they responding to?

And what are the cast and crew of the new Amazon Prime streaming series responding to? In July, I visited the set, just as they were wrapping up the first season – and, yes, I got a cameo. Now I have seen the first season in its rough cut form, and it’s magnificent.

REACHER is played by the actor Alan Ritchson, and he totally nails it. Roscoe, Reacher’s temporary partner in the story, is played by Willa Fitzgerald, and she’s fantastic. But Alan is 38, and Willa is 30. Millennials, for sure. Why are they so into my Boomer One fantasy?

And above all, how is my brother Andrew so good at it too? I’m stepping back from the series, we’re working together for a few books, and then he will take over solo.

The future depends on him. He’s much, much younger than me – a late and unexpected addition to the family. A “bonus issue”, according to our accountant father. He’s a definite Gen X member, whose earliest childhood memories must be from the 1970s and 1980s, when the post-war consensus was well and truly over, and the world was a completely different place.

Yet he gets Reacher perfectly, just like the 101-year-old lady, and the nine-year-old boy. Why and how? There must be a timeless element in there somewhere.

I made Reacher an ex-army officer, because I was interested in dislocation and alienation, and I had noticed that people who have spent their lives in the military have trouble adjusting to civilian life. It’s like moving to a different planet.

Reacher’s transition from the rough, tough world of an army cop makes him a fish out of water in normal life. His response is to be happy about that.

He never settles down. He remains detached, uneasy, bemused, unconvinced – and always alone.

That last part came at a cost. Most series heroes have partners, friends, jobs, houses or apartments, favourite bars and restaurants, cars, bills, neighbours, family, and even dogs and cats. Because most series are soap operas – and I don’t mean that in a derogatory way at all. In fact, I’m a little envious – soap operas are incredibly powerful narrative engines.

I saw that up close – they put food on my table during my ITV years. But I turned my back.

Clearly I was tapping into an even more ancient storytelling tradition – the noble loner, the mysterious stranger, the knight errant, the kind of guy most recently seen in Westerns, like a Zane Grey novel, or the movie Shane, or any Lone Ranger episode: a lonely, embattled community has a problem; a mysterious stranger rides in off the range, solves the problem, and rides off again into the sunset.

That’s a character who has been around forever, in one form or another, for thousands of years, just about everywhere. Like Robin Hood, for example, who I always assumed was a part-true, part-myth, partfable guy who had a beef with the Sheriff of Nottingham about 800 years ago.

But no. He turns out to be a go-to character that was reinvented over and over again, in every culture with a storytelling tradition, whenever necessary, which was frequently.

Times were usually hard. Why else would he be reinvented so often?

People everywhere desperately wanted that character to exist.We still do.Wouldn’t it be great? Whatever specific problem you have, suppose a quiet, competent person showed up at your door, fixed the situation for you, and then left again, with a shy smile and a tilt of the chin?

Plus, people everywhere want to be that character. I wasn’t born yesterday, but even so, I believe most people are decent and full of you the goodwill. They want to help. They want to fix your problem. But they can’t.

They’re intimidated, or powerless, or afraid of the consequences. So they live with a constant buzz of frustration. They want to do the right thing, but they’re prevented.

It’s the same if you’re a victim. The same buzz of frustration.

In the real world, if your car gets stolen, you’ll never see it again.They’ll never catch the guys. If your house gets burgled, you’ll never get your stuff back. The cops might not even visit. No closure.

On page or screen not afraid anything, can do right thing’

You read books and watch movies and TV to be part of the opposite. You get your car back. If it’s a Reacher book, the guys who took it get a smack in the head. On the page or the screen you’re not afraid of anything. You can do the right thing.

That’s Reacher’s appeal across the age range. Everyone, whoever they are, young or old, has a problem. Whatever it may be. Reacher is the modern version of the guy who solves it. He’s the guy who always does the right thing, no matter what. He’s the heir to a long line of storytelling forebears. A grand 1,000-year tradition.

Or maybe more than that. Maybe 100,000 years ago we sat around on sandy cave floors and listened to tales about people who never backed down and always did the right thing.

The concept has been road-tested through all of history. Can’t go wrong. Except it can, in a couple of ways, and this is whereAndrew is very important. The first problem for a character like Reacher is motivation. Doing good is all very well, but why?

Is he some kind of a creepy saint, or what? He needs a solid reason to get involved, or else the story will feel sugary and hollow and inauthentic.

The second problem is the bad guy. Who should he be? Maybe a maniac planning to explode a nuclear bomb in the heart of downtown. But then, what would you do for the next book? A real maniac planning to explode two bombs? Then three, then four?

I don’t like that kind of mindless inflation, and I don’t like that kind of villain. And, deep down, I don’t think readers like them either. It’s not real. Yes, everyone, young or old, has a problem, but very few real people are concerned about nuclear conflagrations on the High Street.

The problems real people want solved are personal and often emotional.

OFTEN they don’t sound like much, but they burn you up. Maybe disrespect, at work, or in a transaction. I always tried to solve both those problems by remembering people who made me cross. What was it about them? I tried to analyse what I felt, until I isolated whatever toxic brew it was that set me off.

Then I could build a character out of that and I could trust Reacher’s reaction to him to be human and authentic, because mine had been, in the first place.

This is partly why it’s so important to me that it’s Andrew continuing the series. We think the same. We’re wired the same. The same things make us cross.We both hate laziness and incompetence.We both hate bullies and smug, patronising people, and therefore smug patronising bullies worst of all.

It’s a finely tuned instinct. Sometimes over the dinner table Andrew will recount a conversation, or an incident, and the others at the table won’t catch it, but I’ll hear the trigger, and I’ll understand that’s a battleAndrew will win, if it takes him half an hour, or the rest of his life.Which is great. It’s that kind of all-or-nothing commitment Jack Reacher needs to stay believable, and that kind of human reaction we need for him to connect with real people’s problems.

That’s why I think it’s perfect that Andrew is continuing the series. He’s not going to change a thing, because he can’t. Not if he writes from the heart. DNA wins in the end.

Better Off Dead by Lee Child and Andrew Child (Bantam Press, £20) is published on Tuesday. For free UK P&P, call Express Bookshop on 020 3176 3832 or click here.