Brooke Greenberg, who has died aged 20, was the nearest thing in science to a real-life Peter Pan; she stopped growing while still a baby and remained, in both physical and mental terms, at the level of a toddler.
Her condition, named Syndrome X because doctors simply did not understand it, is thought to be unique and raised hopes that, by studying her DNA, scientists would gain new insights into the mysteries of ageing and even develop new therapies for diseases associated with the elderly, such as Alzheimer's and Parkinson's.
Yet, although some speculated that a genetic mutation must have switched off her ability to mature, a leading genetics expert found that there were "no apparent abnormalities in her endocrine system, no gross chromosomal abnormalities, or any of the other disruptions known to occur in humans that can cause developmental issues". As a result, she was never diagnosed with any known disorder that would help to explain her condition.
The third of four daughters, Brooke Greenberg was born prematurely on January 8, 1993, in Baltimore, Maryland, weighing about four pounds. Doctors had already become concerned by her spasmodic development in the womb, and it soon became clear that she was far from normal.
Born with a rare condition called anterior hip dislocation, soon after birth she had to have a major operation and be placed in a cast.
Brooke's early life was marked by a series of medical emergencies. On one occasion, seven holes in her abdominal wall had to be repaired. On another, because food kept entering her windpipe, she had to be fitted with a gastric feeding tube. She also suffered strokes, seizures, ulcers and breathing difficulties.
Aged four, she fell into a 14-day coma and doctors diagnosed a brain tumour the size of a lemon. The diagnosis turned out to be wrong and Brooke woke up – but not before her traumatised parents had already made preparations for her funeral, buying a coffin and consulting the rabbi.
By this time, Brooke Greenberg had stopped growing in the conventional sense, never gaining a centimetre or putting on a pound, despite early attempts to boost her development with growth hormone. Yet parts of her anatomy continued to mature.
While her facial features remained unchanged, and she kept her baby teeth and remained the size of a one year-old, in biological terms her bones matured to those of a 10-year-old child and her hair and finger nails grew normally.
Richard Walker, a university profession in Florida who led a research team looking into her case, found that the development of her various organ systems, like the digestive tract, was "disassociated", with different parts developing at different rates, as if they were not a unit but parts of separate organisms.
For 20 years, the family, eventually including her younger sister, changed Brooke's nappies, fed her, rocked her to sleep and took turns to look after her. She learned to pull herself up in her cot, crawl across the floor and whizz along in a specially adapted baby-walker. Though never able to speak, she would smile at people she recognised, giggle when tickled, do finger paintings when given paper and paint pots, and experience ordinary human emotions.
Her parents recalled that when her younger sister was born she would cry with jealousy until she too was picked up. As a teenager, they claimed, she began to show a rebellious streak. But that was as far as it went.
The Greenbergs went to specialist after specialist in search of answers that never came, though in recent years it appeared that her health might have stabilised.
The cause of her death has not been disclosed.