How do you know when it’s time to slow down? I’ve been wondering that ever since I decided to live to 100. Sure, I may not make it; but if I do I expect exercise to play a huge part, alongside a healthy diet and (seriously, kids, this really isn’t rocket science) evidence-based medicine rather than wellness influencers and coffee enemas.

Bone density, muscle mass and cardiorespiratory fitness all decline as we get older; mobility and balance also take a hit. Exercise can slow or reverse most of this, and help protect our brains. It doesn’t guarantee a long and independent life but it does make it more likely.

And it can make you happier, though I only realised this after decades of abusing and undervaluing my body. I was in my 40s before I discovered how much I could enjoy hiking, swimming and skiing; my 50s before I learned to love running; my 60s before I got into yoga. I know it sounds ridiculous, but after more than 22,000 days on Earth I’m just getting started.

I hate to think I might already be too fragile for anything more hardcore like gymnastics, CrossFit, or – let’s dare to dream – cliff diving or parkour. I watch videos of twentysomethings doing handsprings, leaping from roof to roof, generally giving the finger to gravity, and all I can think is: “I want some of that!”

Then I remember I’m 61, with a 61-year-old’s muscles, joints and bones. If I did try it, how long would it be till I tore, sprained or broke something?

Actually, that’s the wrong question. How long would it be till I tore, sprained or broke something else? My right knee has been swollen, weak and tender for months, thanks to a damaged meniscus. This C-shaped bit of cartilage is supposed to act as a kind of shock absorber, but for me it’s more of an agony generator. It hurts to run, climb a lot of stairs or even sit for a long time. Yoga’s out too, because I keep getting swept up in the flow and bending or sitting on something I really shouldn’t.

At least part of my knee trouble is down to wear and tear. Every swelling, throb and cramp drums home the fact that the older you get, the harder it is to avoid injury – and to recover from it. I’m not gonna lie – there’s been times when it’s made me sad with a capital S.

Then I remember I can still walk for hours without any problem, and use both arms. I can even give my knees a good workout if I do it carefully. In fact, I have to: my rehabilitation plan includes endless squats, raises, deadlifts and stretches to strengthen my leg muscles.

My physiotherapist, who is also a runner, encourages me to exercise in any way that doesn’t hurt me. So I’ve had a lot of fun with calisthenics, where you’re mostly working your upper body on rings and pull-up bars, and handstands, where you don’t do much with your legs except a bit of kicking.

It helps that any good instructor will “regress” exercises as needed. That’s fitness-speak for “make easier”. Deborah Lee is a personal trainer based in Grantham in Lincolnshire. She is the same age as me, and used to dealing with injuries and older clients. “You and I come from a generation where you had to ‘hit it hard’ all the time,” she says. “Unless you were bright red in the face and your veins were bulging, you weren’t working hard enough. We now know different.”

***

Anyway, this is the mix of frustration and ambition that has brought me to a converted railway arch in east London, where I am heading nervously towards a mini-trampoline. I’m hoping that when I bounce off it I will fly high enough to manage at least half a forward roll before I smash down on the crash mats that have been positioned to break my fall. If you know anything about gymnastics, I think you’d call what I’m attempting a front tuck.

I have never done anything similar – not even as a child. I don’t remember a single bounce on a trampoline, and was never one of those kids who could cartwheel. But everyone at the Over Gravity gym acts as if it’s the most natural thing in the world. When I talk to co-founder Matteo Cara before my first “tumbling” class, he tells me I could still learn to backflip.

As for my dodgy knee, Cara’s got two of his own. But at 41, he’s still limber enough to do double backflips every time he trains. “I’m not too old to have fun,” he says. “I’m like a kid with an ice-cream.” He kindly spends half an hour teaching me yet more exercises for my knee – and showing me how I can strap it up for extra support. I’m both grateful and inspired.

Alan Cooper, 65, has been coming to Over Gravity since he was in his late 50s. “It’s the most worthwhile, most satisfying thing I’ve ever done,” he says. “I wish I’d taken it up years ago.”

How did he get started? “I asked Matteo to teach me how to do a roundoff back tuck before I was 60 – and he did.”

I have no idea what Cooper is talking about, so I find a video online and have a WTF?! moment. I don’t have the language to describe a roundoff back tuck, but it seems to contain something a bit like a cartwheel, and a sort of spring up, and a backflip, and … look, just watch the video here and be amazed.

The tumbling class itself is led by Mitchell March. He’s decades younger than me, like my 11 fellow beginners, but I’m encouraged that no one does a double take when they see me. Maybe we’re all feeling a little out of place. Gymnastics in the UK is so dominated by kids’ classes that when I headed off to Over Gravity my wife told me to “have a nice time at Tumble Tots”.

We warm up with some running, skipping, jumping and arm-waving (these are not technical terms), plus some shoulder and wrist exercises. I’m fine with all the bouncing, thanks to the sprung floor that softens every impact. I’m also at home with the handstand practice. As usual, I struggle to stay up for more than a couple of seconds, but one of the many nice things about gymnastics is the proliferation of big thick pads. There’s always somewhere soft to land, then up you go again …

There are forward rolls too, which most people can do, unlike the final stage to prepare us for the front tuck – cartwheels. While almost everyone else is cartwheeling from one end of the gym to the other, I’m struggling with a regression designed for total klutzes. It doesn’t make sense. Every time I put my hands on the ground and prepare to put this leg here and then that leg there, I find myself putting that leg here and then this leg there. At times I’m actually going backwards.

The more I work out, the more I realise the importance of proprioception – knowing which bit of your body is where, and being able to move that bit rather than a different one. But mine’s not getting any better. Instead, I’m gradually training a nation of coaches that if they want me to move my hips, say, rather than my ankles, they’ll have to tap me on the right bit instead of just tell me what to do. All I hear is: “Now you need to move your blahblahblah.”

After my cartwheel shame, the news that we are about to move on comes as a relief – until March demonstrates that front tuck. As he runs towards the trampoline, bounces high, then rolls forward in mid-air to land on his feet, there are gasps and nervous giggles. Everything so far has seemed achievable, given enough time and application (12 months till I master the cartwheel?); this seems aspirational. It’s practically flying! It’s magic! It’s terrifying!

But you know what? Even at 61, with a faulty knee and a stomach full of butterflies, it’s not impossible. I run, I bounce, I spin, I crash down on my back, time after time after time. I almost land on my feet on a couple of occasions, which is gratifying, and I 100% land on my head and face, which is less so. I don’t lift my hands high enough when I jump, according to March, or sweep them down hard enough as I flip. I do almost everything wrong, and I’m about as graceful as a sack of potatoes, and yet it still feels great.

I get chatting to some of the other students afterwards and everyone is amazed at how quickly we progressed, and how much we enjoyed it. It feels so natural. I book another class, and then a third. And then, during the photoshoot for this article, I take the opportunity to get some more tips from Cara. Look how high I got! Sometimes I love my job.

***

Is there anything that would stop me doing this regularly? Only something that my calisthenics teacher Lucy Joslin said to me. She is one of the most physically fit women I’ve ever met. But, at 50, even she’s thinking about ageing. The older you get, she says, the more you have to be selective about what you do. You build muscle more slowly than you used to, and you lose it more rapidly. It takes you longer to get over injuries. It’s easy to spread yourself too thinly, so that any progress you make in your new favourite activity comes at the expense of what you were doing, and maybe loving, before.

In short, if I want to become a regular at Tumble Tots, maybe I’ll have to give up something else.

Ugh. Choices. I guess it’s good to still have them.