It seems like Gabby Williams has been a newborn forever, and she has. She still wears diapers and nurses every three hours.
But the little Montana girl with the long hair is 6 years old. As each of her younger siblings were born and grew into toddlers and then developed into older children, she stayed the same.
A normal 6-year-old would weigh an average of 46 pounds and be about four feet tall. Gabby is 10 pounds and only about 24 inches long.
"She had all her medical tests when she was first born and they couldn't find anything," said Gabby's mother, Mary-Margret Williams, 37, who lives with her family in a suburb of Billings, Mont.
Her condition -- so rare that there is no name for it -- will be showcased in "My 40-Year-Old Child," a one-hour TLC documentary that airs Sunday night at 9 p.m.
The film follows Gabby and her family, as well as Nicky Freeman, a middle-aged Australian in the body of a 10-year-old, as they try to unravel the medical mystery.
They are Benjamin Button children -- only about a half dozen of them in the world, who age only one year for every four in the life of a normal human being. And so far, doctors can find nothing wrong with them.
Their chromosomes are normal, but they all have cognitive deficiencies. Gabby is blind and will never speak.
"She cries when she is hurting and sometimes smiles," said Williams. "But there's not a whole lot of communication."
"She is definitely very slow, but she knows when mama and grandma are holding her," she said. "She comforts to people around her. She knows her sisters, who have watched her quite a bit and listens to them play. We have a wild bunch around here."
Williams and her husband, John, have three other children -- Sophia, 7; Anthony, 4; and Aleena, 3. And she is expecting another girl in March who, so far, has shown no abnormalities.
"They watch me very carefully because of Gabrielle," said Williams.
Doctors didn't think Gabby would live long after she was born in 2004.
"She had a real scary birth," said Williams. "The doctor [told] me that she wasn't breathing. It took the whole crew to get her going again. I thought she was OK for awhile [and] then she wouldn't eat. ... They sent me home and said, 'I don't think this baby is going to make it for much longer.'
"We took her home and decided to love her as she is," said Williams. "Gabby is still with us today. She is tougher than most of us."
Dr. Richard Walker of University of South Florida College of Medicine, who has been fascinated by the topic of aging since his undergraduate days, is comparing Gabby's genetic code to another a 19-year-old girl, Brooke Greenberg of Baltimore, who has the same condition.
He wants to figure out not only what is wrong with these children, but learn if others in the family could pass on genes for this bizarre disorder.
Their bodies do not develop as a coordinated unit, but as independent parts that are out of sync, according to Walker. No known genetic syndromes or chromosomal abnormalities can explain why.
Brooke still had baby teeth at 16 and her bone age was estimated to be more like 10.
"There've been very minimal changes in Brooke's brain," Walker told ABCNews.com in an interview in 2009. "Various parts of her body, rather than all being at the same stage, seem to be disconnected."