geing has changed, hasn't it? There's a generation of middle-aged and perimenopausal elder millennials and xennials who refuse to do it. We're young enough to be part of an online world that puts a premium on appearance, too old for such narcissistic self-branding to be achieved without unsustainable effort. Dating apps and social media connect people across generations, so it's easy to become confused about how old we actually are. Here's a tip: ask your knees.

This brings us to Youth (from Monday 21 September, HBO Max), Sharon Horgan's dramedy about being 50. That perky title is a quip about the decade being known as "the youth of old age". Horgan is Alex, a divorcee in her sandwich years, caring for teenage son Ruarí and ageing parents. There's a lot going on in her life, and some doom. She's falling forwards in a semi-successful career as a literary agent, getting back into dating under pressure. She needs someone to fall in love with her immediately, as she points out to her best friend. Not for romantic myth-making, but "so there's a bit of time with the present version of me, before the 'old me' is here".

I've made it sound not funny; it's very funny. The world of publishing is ripe for satire, and the show wields more skewers than a kebab shop. On the walls of Alex's office are posters for prestige titles: Life After Death – A Memoir of Guilt, Babyfingers and Where the Daylight Falls. I've seen all of them on the Richard and Judy Book Club. And then there's Goblin Seed.

Goblin Seed is a ludicrous romantasy in the Twilight or Fifty Shades of Grey mould. (I think that title is a pun, too.) It's written by one of Alex's former authors, a smug creep who self-published after rejection, and has since enjoyed runaway success with his horny, badly written prose ("The goblin seed was gold, and soon the sheets were glittering"). Tasked with wooing him back, every interaction has Alex looking as if she wants to throw up in her mouth.

Youth fires shots across the creative arts. Ruarí's dad, Lorcan, is an actor. Not quite absent, but so self-involved he's not so much phoning it in as texting. He's about to move to Canada to film a lucrative "spaceship show", and has re-recorded his answer-machine message in its alien language, while feeling above the project. He's also dating the young landlady he met when he moved out of Alex's. Oh God, the humanities.

Thankfully, Ruarí has a number of unsuitable father figures to choose from. There's Alex's ex, Patrick, a handsome and "institutionally single" 47-year-old who specialises in sending mixed messages. There's older love interest Michael, played with generous sensitivity by Tim McInnerny. He's stable, kind, attentive. But Alex can't stop staring at his loose chin, and ducks his kisses like Neo dodges bullets.

Sex scenes in Youth are the antithesis of Hollywood gloss, hilarious and penetratingly insightful. "I don't know you well enough to be selfish in bed," Alex confesses to one lover. How has the gap between sex and intimacy widened in her lifetime? It's easy to kid yourself about this at 25, twice as hard at 50.

Horgan's gift is vérité. The friend groups and workplaces are diverse in a way that's London-real, rather than box-ticking from above. Youth is mordantly funny, yet never betrays character. As you'd expect from the creator of Motherland, it's superb on mum stuff. Alex often mutters the word "dick" to herself when Ruarí is being particularly teenage, yet their love is clear. He's played by Aran Murphy, son of Cillian, who delivers a beautifully unforced performance. At points I felt as if I were watching Boyhood.

Then there's the parental subplot, which could carry a series alone. A million miles from saccharine depictions of elderly people, Youth shows the uninspiring, Sisyphean grind of care-giving, which is mostly navigating confused, false-crisis phone calls, sibling resentment and travelling 25 miles to demonstrate the position of the volume button on an iPad. However much we deny our age, life has its own plans. Thank God for Sharon Horgan: chronicler of all our journeys, like a foul-mouthed, funnier 7 Up. This painfully relatable, brilliant show made me feel a bit tired at times. Then again, I'm always tired.