So, she truly was Leonardo's labour of love

The discovery of another version of da Vinci's masterpiece shows that La Gioconda took him 16 years to paint

She has been attacked, parodied and gawped at for 500 years, but still the Mona Lisa keeps smiling. Now, she has suffered perhaps the ultimate impertinence, as curators have questioned her age.

It has always been thought that Leonardo da Vinci took three years to paint his portrait of Lisa Gherardini, nicknamed La Gioconda, from 1503 to 1506. But evidence has surfaced that he actually took 16 years, and that he was still dabbing at her when he died in 1519. This would place the portrait in Leonardo's late period, and would explain why it never belonged to Francesco del Giocondo, the sitter's husband, even though he commissioned it.

The evidence is so compelling that the Louvre has changed its official catalogue listing. And it's all thanks to the extraordinary discovery of a second Mona Lisa at the Prado in Madrid in February.

There are several known copies of the Mona Lisa dating from the 16th and 17th centuries, completed after Leonardo's death; the Prado work was always thought to be one of them. But when conservators peeled back a layer of black overpaint slapped on in the late 18th century, they discovered a delicate Tuscan background, just as in the Louvre original.

In addition, the Prado work was thought to be painted on oak, a wood hardly used in Italy at the time. This meant it was classified as by a northern European artist. But on examination, experts discovered the panel is walnut, which was used in Italy.

And, most sensationally, infrared reflectography images of the Prado copy were taken, showing the work's underdrawing. They were compared with infrared images of the Louvre original, and conservators saw the underdrawings were strikingly similar, leading them to conclude the portraits were painted at the same time, side by side.

When the Prado's technical specialist, Ana Gonzalez Mozo, looked more closely at the background, she noticed it bore a likeness to a Leonardo study of rocks, belonging to the Queen. Martin Clayton, the Queen's senior curator, has dated that sketch as being from 1510-15. After examining all three works, curators at the Prado and the Louvre agreed that the background rocks in both paintings are based on this drawing, meaning Leonardo could not have finished the Mona Lisa in 1506, as previously calculated.

The new theories add yet another layer of intrigue to the Mona Lisa's colourful history. Leonardo started the commission in 1503, when he was living in Florence. Giorgio Vasari, a contemporary and the author of Lives of the Artists, an art historian's bible, recorded that "after [Leonardo] had lingered over it for four years, he left it unfinished". In 1516, Leonardo was invited to work for King François I at Clos Luce, a chateau in Amboise, west of Paris, and he took the Mona Lisa and two other paintings with him.

When he died, the picture passed to his assistant and lover, Salai, though it was subsequently acquired by the king. Inherited by successive French kings, it hung at the palaces of Fontainebleau and Versailles until the revolution, when it passed to the Louvre. Napoleon is said to have had it on his bedroom wall at the Tuileries Palace. But it took a bizarre theft from the Louvre in 1911 to catapult the Mona Lisa to international fame.

Images of the stolen picture were published in newspapers and magazines all round the world. It remained missing for two years, becoming a source of embarrassment to the police and a cause célèbre. Tourists would visit the blank space on the wall where it used to hang. The poet Guillaume Apollinaire, who had once called for the Louvre to be burnt down, and who had a record for handling stolen goods, was thrown into prison. He tried to accuse Pablo Picasso, who was questioned, though both were eventually cleared. The actual thief, Vincenzo Peruggia, was a Louvre employee who simply unhooked it and walked out with it under his smock. He kept it hidden in his Paris flat, and was caught when he tried selling it to the Uffizi gallery in Florence. As an Italian, he said, he felt the painting should be returned to Florence. He was praised for his patriotism and given six months in jail.

The revised dating of the Mona Lisa has been acknowledged for the first time in the catalogue of the Louvre's current exhibition, celebrating the restoration of another Leonardo work, The Virgin and Child with St Anne, which runs until June. A new label is now being made for the Mona Lisa. And the Prado copy, which forms part of the Louvre exhibition, is to be taken to the Mona Lisa for a few hours, so that experts can see the pictures side by side. This could throw up yet more theories and revelations.

But for now, all that's certain is that the Mona Lisa continues to fascinate, as anyone who has pushed their way through the crowds to see her will tell you. And that, 500 years on, she remains as enigmatic as ever, and will do as long as her lips stay sealed. No wonder she is smiling.