A 20-year-old woman who never grew beyond the size of a toddler has died.

Throughout her life, Brooke Greenberg baffled scientists. Her DNA showed no abnormalities in terms of aging and her family had no history of developmental problems. And yet she never developed past the stage of a toddler either physically or mentally. Today, doctors simply refer to her condition as Syndrome X, with some experts suspecting that the decelerated growth begins in the womb.

Though she was born prematurely at just 4 pounds, it was not immediately clear there was something wrong developmentally, Time reports -- that is, until her younger sister outgrew her and began talking even as Brooke remained silent.

According to ABC News, Brooke went through a series of medical crises during her first six years, including a brain seizure followed by what was diagnosed as a stroke but ultimately left no visible damage. When she was four, she slept for 14 days, at which point the doctors diagnosed a brain tumor. The Greenbergs had bought a casket when they got a call and heard she was awake and doing fine.

"She overcomes every obstacle that is thrown her way," Howard Greenberg told ABC in 2009.

When she was five, everything but her hair and her nails stopped growing.

Richard Walker is a retired medical researcher from the University of Florida Medical School who now works at All Children's Hospital. Walker has followed Brooke's case ever since she was two, during which time he's compared her genetic code to others with the same condition.

"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."

According to Walker, the bodies of those with Syndrome X do not develop uniformly. In the case of Brooke, she was able to retain her baby teeth even as her bones aged.

"There've been very minimal changes in Brooke's brain," he told ABC. "Various parts of her body, rather than all being at the same stage, seem to be disconnected."

Walker continues to study Gabby Williams, an 8-year-old who weighs 11 pounds, and a 29-year-old man with the body of a 10-year-old.

"My whole career has been focused on the aging process," he said. "My fixation has been not on the consequences but the cause of it."