India Knight’s writing should be considered as one continuous autobiographical project delivered in bite-sized chunks. Whether she’s producing a newspaper column, a novel or, as here, a work of non-fiction, the subject is always herself. Sometimes she goes shopping (The Shops, 2004), sometimes her marriage stalls (My Life on a Plate, 2000), sometimes she wants to lose weight (Idiot-Proof Diet, 2008) and sometimes she needs to rein in the spending (The Thrift Book, 2009). And now that Knight is knocking on a bit – that’s not rude, it’s a phrase she would use herself – she has written a book about what it is like to be within hailing distance of 50.

The experience of reading In Your Prime is a bit like going out for a drink with a friend who loves the sound of her own voice and, you’ve always suspected, isn’t really interested in your opinion. Nonetheless, your friend has got such a way with words that you can’t help nodding along as she holds forth on everything from whether leopard-skin print at 50 really does make you look like a superannuated hooker or if it should be embraced as “the new neutral”. In the course of her monologue, Knight takes in eye creams (not worth it), the importance of never bad-mouthing your children’s stepmother, and why “fun” glasses, far from being edgy, will tip you into Su Pollard territory.

This is not a book to read if you are the least bit twitchy about privilege: the fact is that Knight almost certainly has more money and a nicer life than you do. In the section on how to dress in middle age, one of her straw figures is Hampstead Lady (AKA I Love Culture, Me) who will insist on wearing sculptural Japanese clothing, a Louise Brooks bob and those specs that look like they’re 1960s NHS but are actually from Italy and cost £400. Knight thinks this is a drab asexual look that suits no one. I happen to disagree and, indeed, have leanings that way myself. But what also strikes me is how irrelevant this is to most 50-year-old women, who are more likely to be worried about facing the age-related chop at work (illegal of course, but good luck with proving it).

Still, as Knight says, one of the main points about getting older is not getting hot and bothered about other people’s daftness. So if the fact that she bangs on about women who wear Issey Miyake strikes you as either entitled or snide, then just don’t read that bit. Afterall, the whole point about autobiographical writing is that it draws its vitality from being smelted out of individual experience rather than being tacked together by a nervous committee. Like Tracey Emin, Knight has frequently found herself fighting accusations of narcissism and shallowness. And, like Emin, she has the best defence: her talent, which is considerable. For Knight’s voice – the equivalent of Emin’s eye, perhaps – consistently lifts what could be read as slack indulgence into something sharp, slanted and bracingly unbothered about what anyone else thinks.

It is, in fact, in those places where Knight’s book tries to embrace a public-service remit that it starts to feel slack and stodgy. Her long section on the menopause merely regurgitates material that you could find within three clicks online. And if you have ailing parents, then you probably won’t be turning to In Your Prime to find out about the pros and cons of getting power of attorney or the best private providers of domestic care. You either know this stuff already, and spend your evenings in chat rooms debating the virtues of incontinence pads, or you’re lucky enough not to have got there yet, in which case you’re unlikely to be swotting up on the hell that lies ahead.

Knight is sublime when she sticks to what she knows, or rather to what she thinks. She points out, for instance, that “wearing purple” as recommended in Jenny Joseph’s tiresome poem “When I Am Old” would simply make you look like a “wacky” art teacher, which is something to be avoided at all costs, however old you are. She is also ruthlessly rude about Pointlessly Long Hair on middle-aged women – the sort of lank, greying hank that makes the wearer resemble a knackered mermaid. Equally wrong is Pointlessly Short Hair which is supposed to be gamine, but which actually requires an unfeasibly sharp jawline if you’re not going to end up looking like the governor of a women’s prison.

I am less persuaded by the recommendations for particular goods and services that Knight sprinkles through her book. This is partly because, again, there are so many excellent online resources for helping you work out the benefits of waxing v threading v lasering (and surely by the age of 50 you know this, unless you’ve spent the last 20 years sporting a tash, in which case you’re hardly going to stop now). But it’s also because Knight’s certainty that she knows about what would be good for you starts to grate. She is evangelistic about yoga for sorting out everything from depression to bad knees, whereas I can promise you equally emphatically that it is the work of the devil and should not be tried unless you like feeling clumsy and stupid in a room full of bendy competitors.

One of the best sections in the book is on friendship, which remains as tricky at 50 as it was at 15. Knight recommends not doing a savage cull of people who have started to annoy you, but simply keeping them at a distance via social media, at which she is a dab hand. This wouldn’t work for me – I don’t do social media at all. So here is how I would deal with Knight, assuming she was my friend, which she isn’t: I’ve never met her. I’d agree to meet up in a wine bar, I’d enjoy her wit and bounce as she advised me on necklines, Botox and vaginal atrophy, although I’d slightly resent the fact that she thought she knew best. And then, as I was leaving the wine bar after an exhilarating evening, I’d make a mental note not to meet up again for a bit until I was feeling confident and strong and all those things you’re supposed to feel in middle age but actually mostly don’t.

• To order In Your Prime for £12.99 (RRP £16.99), go to bookshop.theguardian.com or call 0330 333 6846.