Really Old, Like Forty Five, Cottesloe Theatre, London

A demented take on trials of ageing

It can be no bad thing for a work to take on the nature of its subject – a play about war, for instance, that is one long battle-cry – but the adjective suggested by Tamsin Oglesby's play about dementia is "woolly-minded". Simplistic characters potter about, enveloped in a haze of arch fantasy or daffy benevolence, and themes, untended, wither away.

Really Old, Like Forty Five is set at the midpoint of this century, a future in which an increasingly aged population has brought about drastic measures to deal with the elderly and improve their image. But, 40 years on, health policy and a single piece of technology are all that has changed. Different styles of speech, dress, and the rest might have been pointlessly distracting, but, since the play – as some carefully reproduced teenage slang emphasises – inhabits the present, why keep insisting otherwise? The result only points up the author's lack of invention.

Likewise, the split-level set – bureaucrats above, humans below – calls attention to the disparity of tone. While Monroe (Paul Ritter), a comic Fifties boffin in a three-piece suit, is demented in a wild-eyed, fast-talking way, an Alzheimer's sufferer beneath him realistically mixes up family members, forgets what she set out to do a minute before, and keeps repeating herself. Judy Parfitt's Lyn, the sister of Alice (Marcia Warren) and Robbie (Gawn Grainger), finds her forceful intellect no match for her brain's decay, and, sent to hospital, she must, under Monroe's regulations, take an experimental drug.

She is looked after by Mimi, a lovely nurse all in white, including her little wings. Mimi is a robot sensitive to words, tone, and touch whose responses to the old people "convince them they're in a relationship". In one of the play's two painful scenes – in the other, Lyn's confusion makes her puzzled, then tearful, then angry – Lyn's attachment to Mimi ("my friend") so enrages her daughter that she denounces Lyn as a cold parent she has always hated. Suddenly the play admits that, behind the problems of money and "perception" are people who don't want to give good in return for cruelty and indifference.

But these moments are like chunks of meat in a box of confectionery. Are we really to believe that Monroe, described as "a policy official", is alone responsible for mandatory drug trials and home-death assistance? That the metal device on Lyn's head, which looks as if it is giving her a permanent wave, is "applying pressure to the neurons"? That Alice's teenage grandson comes out with such cute remarks as "See, this is the trouble with the older generation – all this me, me, me"? Can illusion be better than reality? the play asks, with a video game of Lyn bashing her enemies and a tortoise in a tank. Should old people act their age? (Robbie dresses like a teenager and chases women who could be his granddaughters.) Alice's leg is amputated and replaced with a sexy prosthetic, in a red high-heeled shoe. All these themes and touches are flung at us in a half-hearted way, making the work seem less like a play than a series of points for discussion.

Anna Mackmin's production is graced by Parfitt's magnificent, heroic Lyn and Warren's seraphic Alice, but the popular favourite is clearly Michela Meazza's Mimi, a droll, purring robot one imagines many viewers would like to bring home to supplant the spouse who hasn't convinced them they're in a relationship. The play, however, is merely a poster for the playwright's good intentions.

To 20 April (020 7452 3000)