Brooke Greenberg is unable to speak, travels in a stroller and still has her baby teeth. Cognitively, she has the estimated mental age of nine months to a year. But, despite the fact that she looks like a toddler, Brooke was born in 1993, meaning that she is 20 years old. Her condition has doctors stumped, but her family says that they would not take a cure even if it was offered, saying that Brooke is not broken.

Brooke's condition is so mysterious, it is called Syndrome X. She stopped growing at around the age of four or five years old, according to her father's estimate. But doctors have no idea what has caused Brooke to remain frozen in time, though she has been seen by renowned specialists. Dr. Eric Icahn, the director of the Icahn Institute for Genomics and Multiscale Biology at the Mount Sinai Medical Center in New York, said to the Daily Mail that she suffers from "no apparent abnormalities in her endocrine system, no gross chromosomal abnormalities, or any of the other disruptions known to occur in humans that can cause developmental issues."

However, scientists believe that Brooke seems to be aging out of sync. For example, according to ABC News, Brooke still has baby teeth, but her bones appear to be about 10 years old. Her mother said in 2009 that she had the personality of a 16-year-old, then her chronological age; her family joked at the time that she rebels.

Brooke's entire life has been filled with medical mysteries, most of which she recovered from seemingly without explanation. She had seven perforated stomach ulcers for which she required surgery. She suffered from a brain seizure, followed by a stroke that left no damage. At four years old, she had a bout of lethargy that caused her to sleep for 14 whole days. When doctors diagnosed a brain tumor, her parents bought her a casket and prepared to say their farewell - only to be told that the tumor had apparently disappeared, she had opened her eyes and seemed to be fine. She was prescribed growth therapy by one doctor, only to find that it did not cause her to gain an inch or a pound.

Now, Brooke is fed through a tube. Because her esophagus is so tiny, it could back up into her lungs, causing pneumonia.

Doctors believe that studying Brooke could reveal the answer of why humans age - and die. Tests have shown that Brooke may have a gene mutation that has switched off her ability to age, and scientists are in the midst of sequencing her genome. Dr. Schadt believes that the findings may have implications for longevity and age-related illnesses like Alzheimer's disease.

In the meantime, researchers have no idea as to how long Brooke's life may be. For now, if Ms. Greenberg is approached in the street, she converts years to months. "So, if someone asked today, I might say, she's [20] months old," she said.