Brooke Greenberg when she was 16 years.
THE father of Brooke Greenberg, a 20-year-old US woman who baffled doctors because she did not age, has spoken of his heartbreak at losing his beloved daughter.
Howard and Melanie Greenberg's daughter Brooke died on October 24 with an unidentifiable and extremely rare condition, Syndrome X, which has presented itself in only a handful of cases around the world and stops children from ageing physically or mentally.
While Brooke was 20-years-old when she died she looked and acted like a toddler.
Mr Greenberg, whose family is from Reisterstown, Maryland, told News Corp Australia that having a child with Syndrome X is a situation that few people can comprehend.
"All the things you take for granted in your life was my reality," he said.
"You don't have to worry when you walk down stairs whether or not your daughter will be breathing or not. That was our reality every morning when we woke up.
"We didn't have the chance to do things that other families do like travelling to New York because we would always need someone to look after Brooke.''
Mr Greenberg said the family had no plans to set up a foundation or become the public face of Syndrome X.
"We just want to go on and live our lives and remember our daughter," he said.
Despite the pain of losing Brooke just last week, Mr Greenberg said he would always be willing to speak to anyone whose child suffered from a similar condition to his daughter's.
"I would like to talk to them," he said. "Absolutely."
Gabby Williams is eight years old, but still looks and behaves as an infant.
Montana girl Gabby Williams has a similar condition to Brooke. She is eight-years-old but still looks and acts like an infant.
Then there is Audenete, a 31-year-old woman from Brazil. While she is unlike Brooke and Gabby in that she can see, hear and function like an adult, she weighs just 13 kilograms and looks like a child.
"You have to remember that what these other people have might be different to what Brooke had," Mr Greenberg said. "They call it 'Syndrome X' because they simply don't know what it is."
Mr Greenberg said that Brooke was "treated just the same as our other children" but that she had given him a greater ability to handle life's hard knocks head on.
"Some days were rocky and others were great but we would always deal with things the very next day once they came up," he said. "That was because of Brooke."
Her death was almost as mysterious as her condition.
Mr Greenberg said the family had no indication that Brooke was about to pass away last week. But in an illustration of just how tight-knit the Greenberg family is everyone was at her bedside when she died.
But can Brooke's mysterious condition hold the secret to eternal youth?
"If we could identify the gene and then at young adulthood we could silence the expression of developmental inertia, find an off-switch, when you do that, there is perfect homeostasis and you are biologically immortal," medical researcher Richard Walker said.
Gabby Williams of Billings, Montana, weighs only 4.8kg and still looks like an infant.
She needs to be cared for as if she is a newborn, with her mother and father changing her nappies and feeding her multiple times a day.
Her mother, Mary Margret Williams, told ABCNews.com that Gabby hasn't changed much over the years. In fact, her skin still feels like a baby's and her hair is still fine-textured.
"She has gotten a little longer and we have jumped into putting her in size 3-6 month clothes instead of 0-3 months for the footies," she said.
Audenete, 31, lives with her father and stepmother on a farm in Brazil.
Audenete, who was one of several people featured on a TLC TV special recently, weighs just 13kg and still has the body shape and proportions of a typical toddler.
She was more than five times smaller than the average woman her age.
Unlike Brooke and Gabby, she could see, hear and function like an adult. Her mind is that of a typical woman her age. It's her body that has fallen decades behind.
"She believes she is a woman," said Audenete's stepmother, Dora.
"She likes to look pretty, to wear jeweller, to brush her hair."
Audenete, 31, from Brazil has the body of a child but otherwise functions as an adult. The condition has baffled medical experts.
Richard Walker is just one expert searching for clues as to why these individuals don't age and what they have in common.
He explained that "developmental inertia," or physiological change, is vital for human growth.
"Without that process we never develop," he told ABCNews.com. "When we develop, all the pieces of our body come together and change and are coordinated. Otherwise, there would be chaos."
"If we could identify the gene and then at young adulthood we could silence the expression of developmental inertia, find an off-switch, when you do that, there is perfect homeostasis and you are biologically immortal," Walker said.
By "biologically immortal" he means people would still die from disease and in accidents, but they wouldn't experience the normal effects of ageing.
"You wouldn't have the later years," Walker said, "you'd remain physically and functionally able."
For Brooke's grieving parents, a "cure" never really mattered.
Melanie Greenberg told Katie Couric she loves Brooke the way she is.
Howard Greenberg added he wouldn't want a cure for her anyway: "She's not broken".
