For his seventh birthday, Jacques-Henri Lartigue was given a camera. In a world awash with disposable cameras, APS cameras, 35mm cameras, digital cameras and endless varieties of camcorder, where everything is recorded in some form or other, it's hard to imagine just how unusual an event that was. For Lartigue was seven in the year 1901, and the camera wasn't a portable gadget with a pop-in film and a built-in light meter and range-finder, but a big wooden tripod-mounted box with an adjustable bellows lens, for which exposures were calculated in your head and made onto big, individual, fragile plates.

These plates were, Lartigue confided to his diary, "wrapped in beautiful black paper, which I was quite wrong to unwrap in the light". Really, he concluded, "it was all too heavy and complicated for a little boy". But within a year the little boy was taking photographs without adult supervision, using that cumbersome camera, and later a succession of more portable and versatile ones, to explore the concentric circles of the world that opened out before him.

Prints from some of his earliest plates are included in the retrospective of his work currently showing at the Gallery of Photography. There is a picture of his parents, a view of his collection of toy cars on his bedroom floor. The surface flaws and irregularities of the process lend these images a certain exoticism, but even so they are striking, and the really extraordinary thing about them is not so much the nominal subjects, but how they capture the childhood perception of space. The spaces are huge, enticing and mysterious, full of possibility.

The young Lartigue might have tired of his new toy, might have found that it all took far too much time and effort and kept him from playing with his brother Zissou and his cousins.

But instead, events took quite a different turn. He took his camera out to play, and the result is a series of brilliant images of movement. He took photographs of his nanny Dudu throwing a ball in the air, of his cat caught in mid-leap, of his playmates running, jumping, tumbling, cycling, swimming, go-carting, sliding and just generally messing around: all banal enough subjects, it is true, but captured with an innocence and instinctive flair that lift them onto another plane altogether.

There are a number of reasons for his inventive perseverance. He was fortunate enough to be born into a well-off family in belle epoque France. But, more, it was an unconventional, exceptionally outward-looking family, with a tradition of involvement in imaginative engineering projects. His father was himself a keen amateur photographer, and Jacques-Henri, from an extremely early age, was a diarist with a Proustian flair and passion for detail when it came to the minutiae of family life. He kept an exhaustive journal and photo album that, when he eventually presented it to the French state, comprised some 250,000 photographs.

It looks as if family life, for the extended Lartigue clan, was a tremendous amount of fun. The mechanical age was in its infancy, and there was a real glamour attached to automobiles and aeroplanes, which were, understandably, objects of endless fascination for the young boy. He records numerous experimental flying machines, from winged bicycles to the family's own gliders, and his photographs of cars, from inside and out, unerringly capture the novelty and excitement.

A car is never just a car, but is always lovingly captioned with its precise manufacturer's and mechanical details, like the wonderful image of Automobile 22HP Peugeot encountering a flock of sheep in Auvergne.

Women were also a favourite subject. He got into the habit of writing ideas for pictures in the margins of his journal. One note reads: "An idea: take a picture of Bichonnade and Mama when they are wearing nice hats." This was the beginning of a whole series of images of fashionably, elaborately attired women strolling in the Bois de Boulogne or at the races at Montreuil and Auteuil (to which Cecil Beaton referred when he was designing the costumes for My Fair Lady.

All of these photos were essentially family snapshots. Lartigue took and printed them, and filed them away in his albums. They had nothing to do with his professional life and were not known or exhibited publicly until the 1960s. Yet, ironically, his fame rests entirely on them. A keen draughtsman, in his late teens he set out to be a painter, studying at the Academie Julian in Paris. Indeed, he went on to paint professionally but, as one commentator later observed, "the keenness of his insight and brilliance of his observation of people, things and places seemed to come to an end when he started to work out his aesthetic problems on canvas."

It is a cruel fact that he never quite equalled the achievements of his early photographs in either painting or, for that matter, photography. If you were romantically inclined, you could argue that this merely reflected Europe's cataclysmic change of direction once the horror of the Great War was unleashed. But as it happens, Lartigue personally went on to have what was by all accounts an extremely happy, fulfilled life, living to the age of 92 and never losing his sunny disposition. Having lived through two world wars, he later remarked, the only injury he'd sustained was a cut on his leg - and that happened when he was on a butterfly hunt.

There are later pictures in the exhibition, and they are very good photographs. His early schooling in the handling of limited, inflexible cameras honed his instincts, though he remained famously offhand about technical matters. When a fellow photographer asked him what technical advice he'd offer to someone starting out he replied: "Fall in love." The real problem with his later work is that he grew up, and once he became an adult, his pictures became conventionally pictorial.

But brilliant photographers of childhood who happen to be children themselves are still a rarity, and we should be extremely grateful that exactly the right child was given a camera in 1901.