Timing is not always everything, but Tamsin Oglesby’s sharp as a tack new play could have been prescribed specifically to accompany Terry Pratchett’s headline-hogging plea for dignity in death by assisted suicide.

But instead of suggesting that old age alone is the cause of amnesia or Alzheimer’s, her play more interestingly broadens the scope of the argument to encompass the whole human condition.

Nobody’s perfect. Vacancy is as much a state of mind in the young as the old, and policy wonks are just as susceptible to tricks of perception as the batty old dear who thinks the hospital’s a hotel and that sex is like pantomime – “silly and rude, but at least it’s only once a year.”

Oglesby’s smart plays usually stem from one big idea: the vicissitudes of the beauty business, or the after effects of school bullying or, most recently, in The War Next Door, domestic violence as a paradigm of cultural conflict.

Here, the slippage of two old sisters - Judy Parfitt’s grizzled and combative Lyn and Marcia Warren’s younger and deliciously off-centre Alice - into the health system triggers a comedy of caring that revolves around the slightly obtuse idea that an elderly turtle in the Natural History Museum represents the perennial life force manifest in a new baby’s arrival.

The writing is funny and clever, the acting in Anna Mackmin’s production uniformly superb, but the dramatic texture’s a bit thin, and the play comes across as burningly topical but somehow too diagrammatic.

The old dears have an even older brother, Robbie, whom Gawn Grainger presents as a cantankerous old sod, head-butting fellow customers in the bar of the National Theatre (you know the type) and changing his girlfriends with the regularity of his increasingly youthful wardrobe.

And Paul Ritter’s manic policy official, Monroe, leads a research team (Paul Bazely and Tanya Franks) glued to their charts and computer screens devising pavement strategies and euthanasia directives as the population becomes top heavy with the over-sixties.

Lyn’s daughter (Amelia Bullmore) and Alice’s grandson (Thomas Jordan) are no less peculiar than the oldsters, while Michela Meazza’s robotic nurse proves that curing and caring is often more bizarre than natural disintegration.

- by Michael Coveney