Little American girl, Gabby Williams weighs 5kg– just a little more than a newborn. She has the face of a baby, the body of a toddler and wears nappies.

But Gabby is actually nine years old. As everyone around her ages, she has stayed the same. Gabby has a condition so rare that experts still haven’t given it an official name. She’s been dubbed the real-life Benjamin Button, because, like the film starring Brad Pitt, Gabby has a disorder that dramatically affects her ageing process.

It takes her four years to age just one year – she’s a medical mystery, and doctors believe she could hold the key to biological immortality.

A rare condition Gabby lives in America with her parents and five siblings. She’s the second-oldest child, but the smallest of the brood.

When she was born, Gabby was purple and limp. Tests revealed she had a brain abnormality and her optic nerve was damaged, which has left her blind.

She had two heart defects, a cleft palate, and an abnormal swallowing reflex, which meant she had to be fed through a tube in her nose.

Doctors told Gabby’s mum and dad, Mary-Margaret and John, to expect the worst.

‘They knew something was seriously wrong,’ says Gabby’s mum.

‘But my husband and I hoped it was just a feeding problem and that it wasn’t going to be a big thing.’

Chromosome tests came back negative for well-known genetic conditions – doctors had no idea what was behind Gabby’s problems.