The most shocking work of art on show in London right now is a painting that’s more than 500 years old.

A woman poses behind a stone parapet in golden Venetian light. She points at herself. In her hand is a banner that reads “Col Tempo” – with time. Her clothes are ragged and so is she. Time has cut lines into her face and turned her skin to crumpled paper. She opens her mouth to reveal worn stumps of teeth as she looks back at you with grim knowledge. Dry grey threads of hair complete the portrait of decay.

Old age is not repulsive. This painting just shows the facts of human ageing. It is disturbing because it uses the conventions of Renaissance portraiture. The woman’s pose, the light that bathes her, the exquisite style of the delicate brushwork, all evoke the beauty of Renaissance art – but this painting says those conventions are lies. Art is a fiction. Reality is decay. Time is death.

Giorgio da Castelfranco, nicknamed Giorgione, painted La Vecchia 1508-10. Leonardo da Vinci had just painted the Mona Lisa. Giorgione’s magnificently defiant, self-knowing old woman is the Mona Lisa’s antithesis – no mythic beauty but a real person facing life’s more brutal truths. She is not only old but evidently poor. Her clothes tell of social desperation. The Renaissance is supposed to be about loveliness and luxury. Who was this artist who portrays human weakness with such accuracy and compassion?

Giorgione, the lost hero of Renaissance art, died at just 32 on the Lazzaretto Nuovo, an island where the Venetian Republic quarantined people exposed to plague. According to his first biographer, Giorgio Vasari, writing in 1550, this tragically early death resulted from the wild lifestyle of an avant garde icon. A brilliant musician as well as painter, Giorgione seduced ladies by playing his lute under their balconies until he finally caught the plague from one of his lovers.

Giorgione’s Il Tramonto (The Sunset). Photograph: The National Gallery, London

His paintings seem to foresee his own cruel fate. They are perfumed with melancholy, enraptured by the fragility of human beings. Every young man he portrayed looks like a lovelorn poet. The youth in the Giustiniani Portrait (c1497–9) looks at you out of dark reflecting pupils. His pale skin and pink blouse add to the softness and delicacy of his aura. Another poetic young man may be the real-life Venetian poet Antonio Brocardo. He has his hand on his heart as he looks downward in gloomy introspection.

Nothing pleased this artist like escaping the noisy city and wandering the countryside, lute in hand, a woman on his arm. And what he really loved was to get the women to strip off in some tranquil glade. Giorgione revolutionised the female nude. Before him, Renaissance artists were quite decorous in approaching the goddess Venus. Giorgione made nudity carnal, hedonist. He was quickly imitated by other Venetian artists, including his younger friend Titian, who followed him out into the meadows with wine, music and courtesans.

Art of an elder: Giovanni Bellini’s Virgin and Child with Saint Peter, Saint Mark and a Donor Photograph: Photo (c) Birmingham Museums

And that’s the problem with Giorgione: for all the traces of his brilliance that survive, his early death and apparently slow working pace mean that he is mostly an influence, an atmosphere, a lingering whisper of pastoral sadness and passing beauty. His reputation has gradually been eclipsed by two massive artistic personalities: his elder Giovanni Bellini and his successor Titian, who took his innovative approach to portraiture and the nude into a luxuriant stratosphere in a career that lasted so much longer than Giorgione’s.

At times in this exhibition, for all the curators’ attempts to put him back into the limelight, Giorgione seems a lost cause. A roomful of religious paintings is dominated by two big Titians that blaze into your very soul. Was Giorgione just a John the Baptist to the artistic messiah Titian?

No. He was Marlowe to Titian’s Shakespeare. Sure, Titian took art to new heights. But just as Shakespeare could never have written anything as rawly provocative as Marlowe’s plays, it is impossible to imagine Titian painting anything as bleak and unadorned as Giorgione’s picture of an old woman that brings this show to its devastating conclusion. This metaphysical painting of human fragility is the work of a true genius who deserves, even from his few surviving relics, to be revered as one of the giants of European art.