When Brooke Greenberg died in Maryland last week at 20, she had never developed beyond the size of a toddler.
She suffered from a rare genetic condition that prevented her from aging. She slept in a cot, was pushed around in a stroller, and doctors estimated she had a cognitive age of between nine months and one year.
Brooke was one of about a dozen children in the world who have what some call syndrome X -- a kind of Benjamin Button disorder that prevents them from aging.
She was born a month premature to Howard and Melanie Greenberg of Reisterstown, Md., and weighed only 4 pounds. For most of her life, Brooke was under the care and attention of doctors, with orthopedic problems, stomach ulcers, respiratory distress and other conditions.
The Greenbergs consulted numerous doctors and specialists who were all baffled by her condition. DNA studies turned up no abnormalities in the genes associated with aging, her parents had no history of abnormal development, and her sisters were normal as well.
"Her unusual condition raises intriguing questions about how people age,'' wrote Alexandra Sifferlin at Time.com, "and whether it's possible to interrupt normal development in any way to bypass what seems to be the inevitable march toward old age.''
"She literally is the fountain of youth if you think about it," her father, Howard, told NBC in 2005. He and his wife consented to having their daughter's DNA sequenced by researchers to learn more about longevity.
In 2009, when her family was interviewed on ABC's 20/20, Brooke weighed 16 pounds and was 30 inches tall. She didn't speak, but she laughed when she was happy, and clearly recognized her three sisters: Emily, now 26; Caitlin, now 23; and Carly, now 17.
Earlier this year, Brooke and her family appeared on NBC to discuss her condition. Her parents explained how their 20-year-old daughter could not talk and still had her baby teeth and travelled in a pram.
"From age one to four, Brooke changed. She got a little bit bigger. But age four, four to five, she stopped," her father said.
Only her hair and fingernails grew.
Richard F. Walker, a retired medical researcher from the University of Florida Medical School who now does his research at All Children's Hospital in St. Petersburg, has followed Brooke's case since she was about 2.
"In some people, something happens to them and the development process is retarded," he told ABCNews.com earlier this year. "The rate of change in the body slows and is negligible."
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.
"There've been very minimal changes in Brooke's brain," he said. "Various parts of her body, rather than all being at the same stage, seem to be disconnected."
Sources: ABCNews.com, Sydney Morning Telegraph, NBCNews.com