So the Big Bang Theory’s Jim Parsons confirmed quietly yesterday, at the end of an interview in the New York Times, that he’s gay and has been in a relationship with a man for ten years. Our reaction: no great surprise, no big deal. What did leave us stunned is the fact that Parsons is 39 years old!

OK, so there have been hints that his character Sheldon Cooper and his astrophysicist associates are pushing 30, but they work in a university and spend most of their free time playing video games and eating take-away so to the casual viewer they seem like students.

The thing is, Parsons is lucky enough to look that age in real life. And he’s not alone. Johnny Galecki, who plays Sheldon's roommate Leonard Hofstadter, was working on Roseanne 20 years ago (back in 1992) and looks pretty much the same today as he did then. He also plays a postgrad, yet in real life he's 37.

Indeed, the pair are meant to be contemporaries of their neighbour Penny, played by Kaley Cuoco - but Kaley is only 26 in real life. If her show-mates are willing to divulge the secret to looking so young, she needn't worry about Big Bang getting canned in a decade — she'll still be able to get a job in a teen drama.

So, what is the Big Bang Theory's secret of eternal youth? We’d love to know the science behind that…