What is epigenetic ageing, and can we control it — or even reverse it — with diet and exercise?

For centuries, writers and poets have told stories of the Fountain of Youth, said to turn back the clock if you drink or bathe in its waters.

But instead of aquatic alchemy, could we slow the inexorable march of time with broccoli and lentil sludge?

A recent thread doing the rounds on Twitter highlighted tech entrepreneur Bryan Johnson's "Project Blueprint", which he claims reduced his "epigenetic age by 5.1 years in 7 months".