From Jane Fonda to Elizabeth Warren: all hail the rise of the alpha-grannies

As I watched 81-year-old Jane Fonda getting arrested at a climate change rally on Capitol Hill on Friday – the second time in as many weeks, magnificently stylish in her revolutionary-red coat as she was led away by a young policeman – I felt a wave of admiration: the older you get the more kudos there is in standing up for what you believe in, even if you've been doing it your whole life. There's nothing we like more than a gutsy Alpha-gran; from Fonda to Dame Helena Morrissey, 53, one of Britain's most successful businesswomen with a mere nine children and one grandson under her belt, and 71-year-old Hillary Clinton, flirting with the highest office on earth while maintaining a close relationship with daughter Chelsea and looking after her three grandchildren, which she describes as "an absolutely life-changing experience." Senator Elizabeth Warren, running for President next year, has three grandchildren of her own.

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These days high-achieving women don't stop their trajectories when they enter the 'third age.' Granny isn't longing for Red Riding Hood's visit; she's donning her own red outfit and marching out on a protest. Or working 24/7 on her new start-up. Alpha grans don't consider slowing down, still less (heaven forbid) stopping work – instead, they just add cuddling a grandchild to all their other tasks, glad their earnings allow them to buy stylish baby clothes.

I'll admit that back here in real-world grandmother land, Fonda's protesting exploits left me feeling a little jaded. It's fine for you, I thought, swanning around like a heroine – but I'm at home being an alpha-grandmother for real. This week was epic. We had two (seven and three) of our four grandchildren staying while their parents, my daughter and her husband, flew off to celebrate their 10th wedding anniversary in (as fate would have it) Barcelona.

They phoned to tell me of police choppers circling over their hotel, while I was blasted by the chaos of school uniforms, reading books, raincoats etc. But I also wrote three articles and served my elderly parents (97 and 95) a meal. Tiring? The intake of vodka and tonic doubled.

Fonda says she was never very maternal first time around, and celebrated grandmotherhood as "a second chance." But a second chance for what, exactly? If you were heavily engaged with career and fame when your own children were young, do you really want to give it all up to play with Peppa Pig? Alpha-grans might imagine (with a dash of sentimentality) they will change – but will they? I don't think so – and I should know. I just turned 73 and feel more involved with my writing career than ever. My generation is still "having it all."

That means rising to the top of your game in your third act, both professionally and personally. And throwing out the old stereotypes – cosy slippers, a tartan knit – while you're at it. Take the model Yasmin le Bon, who at 54 is far from the cliché image of a white-haired old lady in the corner, peering at her cross-stitch through specs. Early this year, modelling for Calvin Klein, Yasmin said, "It sounds stupid, but I didn't want to look like someone's grandma on the runway. But actually, I am someone's grandma, and I am on the runway, so there you go."

The supermodel said she wears more miniskirts now than in her twenties and wants to cling on to her inner 'rock chick'. And she defies any critics: "I'm sure they say things behind my back, but what I don't hear or see doesn't hurt me." Age has made me similarly resolute in my fashion choices – like all my friends, I wear what I like. Skinny jeans and funky jewellery are my trademarks. And I never wear beige.

Baroness Brenda Hale's wardrobe recently made headlines, too: the 74-year-old supreme court judge's bejewelled spider brooch spawned its own Twitter fan account last month, while Margaret Atwood, who was jointly awarded the Booker Prize last week at the age of 79, has also become a cult hit among women both her own age and those in younger generations. When it comes to reaching the height of your professional prowess, there really is no age limit.

And Helena Morrissey has taken this to heart – she may be on her hands and knees with the plastic toys, but that hasn't stopped her overtly pitching for Mark Carney's job. Grandmothers or not, these days older Alpha-women go on and on. They won't be slowing down soon. Why should they?

I have a friend in her late sixties, a widowed grandmother to four large lads, who went out recently to buy leathers because her new chap is a biker. "The boys can't quite believe their Nan's doing this" she smiled. We've agreed that our own grandmothers were very different; at this age they didn't expect more than domesticity and loving 'service' to the family, whereas we want to continue working and cramming every minute while we can.

What is a real Alpha-gran? The doughty women all over the world who have no choice but to raise their grandchildren – who are often exhausted and worried but just get on with it, doing their best with pride and love.