Everything is in place for the great party. There is just one problem: how to fit 150 candles on the birthday cake.

According to one scientist, Nir Barzilai, this scenarioAn imaginary situation. The word was originally Italian and referred to the plot of a stage drama. is close to becoming reality. He believes that a cheap diabetes drug called metformin could add years to ordinary people's lives.

"Once we prove it, I think it will be an earth-shattering moment for everyone," he says.

Life expectancy today is much greater than it used to be. In the UK it almost doubled between 1841 and 2011.¹

But living longer is of little help if people spend those extra years in bad health. They have a miserable existence, and add to the strain on health services. So scientists are now aiming to increase "healthspan" - the number of healthy years we live.

Part of the challenge is raising money for research. The trial Barzilai wants to run for metformin is expected to cost between £40m and £60m, and take up to six years.

But companies who invest in healthcare usually want quick profits. So scientists are looking to billionaires such as Jeff BezosThe founder of Amazon and one of the world's richest people. to fund their research.

Bezos has already invested millions of dollars in a company called Altos Labs. It aims to take living cells and rejuvenateGive new energy to. them in a laboratory before putting them back in the body.

The worry is that if super-rich pay for research, they will be the ones who benefit from it. The result could be a society in which the gap between rich and poor is even larger than it is already.

But the head of one anti-ageing institute, the Hevolution Foundation, is determined to avoid this. Mehmood Khan says that although it is backed by the Saudi royal family, its aim is to extend everybody's healthspan. "If this is going to be a gazillion-dollars' worth of treatment for a handful of people," he says, "it is of no interest."

Yes: Although it is nice to think that we would all benefit from anti-ageing treatments, it is inevitable that only the rich would. No government could afford to pay for the whole population.

No: The history of medicine shows that treatments that are very expensive to start with become cheaper over time. Anti-ageing drugs pioneered by the rich will eventually become available for everyone.  

Or... It is not in the interests of humanity to make people live longer. It would simply increase the strain on the planet's resources, and on young people who have to look after the older generation.