Diagnosed with a mysterious condition that prevented her from aging, Brooke Greenberg was old enough to drive but lived her life in a crib.

From the moment she was born, Baby Brooke’s parents had reason to fret about her size. She was a month premature and weighed only 4 lb. (1.8 kg), much bigger than the tiniest preemies, but far too small for comfort. For a time, it was easy to hope she would catch up with her peers, but when her younger sister was born and eventually overtook her developmentally — walking, talking and maturing cognitively — it became impossible to deny that something was wrong. For most of her life, Greenberg remained frozen at age 5 — and for most of her life, she was under the care and attention of doctors, with orthopedic problems, stomach ulcers, respiratory distress and other conditions, according to NBC News.

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DNA studies turned up no abnormalities in the genes associated with aging, her parents had no history of abnormal development and all of her sisters (she had three) were normal as well. There are so many unanswered questions about Greenberg’s condition that scientists continue to refer to it as simply Syndrome X. For now, some developmental experts suspect that the slowed development begins in the womb; growth may be stunted for one month, and then catch up the following month. This process may continue even after birth, in a one-step-forward-two-step-back pattern throughout a patient’s life. In Greenberg’s case, however, the entire developmental process stopped when she reached the age of 5. A handful of similar cases have been identified in the U.S. and overseas, and those remain medically unexplained as well.

Greenberg died on Oct. 24, but her unusual condition raises intriguing questions about how people age, and whether it’s possible to interrupt normal development in any way to bypass what seems to be the inevitable march toward old age. Those opportunities weren’t lost on Greenberg’s parents, either. “She literally is the fountain of youth if you think about it,” her father Howard told NBC in 2005. They consented to having their daughter’s DNA sequenced by researchers at the Icahn Institute for Genomics and Multiscale Biology at the Mount Sinai Medical Center in New York City to learn more about longevity; the scientists have inserted her genes into fruit flies to better understand how they affected her development, the Huffington Post reports.

Her funeral was on Sunday in Pikesville, Md.