You don’t have to go far to find supposed ways of delaying the ageing process, from oddball diets to special supplements. But these fountains of youth are all hype and no substance. For now, there are only a few methods that have consistently extended the lives of mammals. Eating less – formally known as “caloric restriction” – is one of these. Rapamycin, a drug originally found in Easter Island bacteria, is another. It can lengthen the lives of old mice by 9 to 14 per cent, and it boosts longevity in flies and yeast too.

But rapamycin has its downsides. For a start, it strongly suppresses the immune system. That is why it is currently given to people who receive new