In one of the most evocative passages in a novel full of description, Margaret Drabble offers a portrait of a child her narrator sees in an airport queue.

This is one of what Eleanor calls "the special babies". No more than eight months old, she writes, "you couldn't miss him or forget him … he was smiling, and making free-range crowd contact, and stretching out his little waving neat-fingered hands to strangers, and responding to their clucks and waves." He was, she reflects wistfully, "supernatural in his happiness".

So, at first, is the pure gold baby of the title. Anna is the child of the narrator's lifelong friend Jess, an anthropologist and a single mum in the days when neither was common in middle-class London. At first, Jess is delighted with her adorable daughter. In time, as it becomes clear Anna is not quite normal - her affliction is not precisely specified, but she never learns to read or write - the girl's life is increasingly constrained, as is her mother's. Neither is unhappy, but there is a cloud over their heads, the unspoken terror of what will happen to Anna when Jess is gone.

The Pure Gold Baby begins in the 1960s, and is gradually brought up to our own times. In recounting her friend's tale, Eleanor, or Nellie as she's known, feels a sense of trespass, as if she's telling a story that is not hers to relate. Maybe she is right. But the fascination the spirited Jess holds for her group of women friends is intense, making her admirable and enviable, despite her troubles. In their younger days, thinks Nellie, if it hadn't been for Anna, they would all have feared for their husbands.

What follows, as one might expect with a Drabble novel, is a mildly comic, undemonstratively tragic account of the past few decades in a shabby-chic postcode of England's capital and the western world at large. Filled with dry asides, on occasion almost aphoristic in its detached perspective on times past, it is a frequently aggravating work that, in following Jess's life over the years, follows the curve of any biography with its ups and downs, its dramas and longueurs.

The core of the novel seems to be motherhood, and how we treat disability and vulnerability. But this is not enough for Drabble's restless imagination, her fierce engagement with the ever-­changing cultural mores of the late 20th century. Incorporating a lifetime's cast of characters, The Pure Gold Baby thus encompasses a shifting assemblage of the disaffected, the depressed and the dysfunctional. Some are severely mentally unwell; others such as Anna are able to be part of ordinary society so long as they are well looked after.

In scenes spanning the decades, Drabble unfurls her doctrine about the treatment of the imperfect, at the same time as returning repeatedly to Jess's area of special interest: children in Africa, who have a condition that deforms their limbs, yet who thrive nevertheless. Out of this interest springs a fascination with missionaries and explorers, notably David Livingstone and Mungo Park. These intrepid Scots represent the extremes of colonial attitudes towards the African continent - Park a child of the Enlightenment, Livingstone a Victorian moralist and improver.

The inclusion of this particular strand adds yet another didactic element to this novel, as if Drabble is as concerned to chart modern political and social progression, and the errors of our forebears, as to focus on Jess and Anna, who are at risk of becoming mere symptoms of their times.

Perhaps mirroring Drabble's butterfly mind, the narrator Nellie's change of focus is often abrupt. One minute the reader is given an eagle's eye view of post-war London, the next the story swoops in on Jess's household, on what they eat, how they dress, who they see.

Innately, insistently nostalgic, The Pure Gold Baby is threaded with comments such as: "We did not know about cholesterol then. It had not been invented," or: "Those were the days before we were told to fear the sun."

The predominant theme emerges as nothing more controversial than ageing itself. On this Drabble is acute, neither sentimental nor in denial, but hovering somewhere between defiance and resignation.

The Pure Gold Baby, as becomes increasingly obvious, has no strong storyline but the unfolding of a portmanteau of lives. This is not without interest, and Drabble's voice is both commanding and conversational. In this she is like a companion who talks too much, but has many intriguing things to say. Very like, one might suggest, her fictional characters.