Brooke Greenberg may be 20 years old, but she remains forever trapped inside the body and mind of a toddler, due to a mystery medical condition that has baffled medical experts for years.



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"From age one to four, Brooke changed. She got a little bit bigger," explained her father, Howard, during the family's appearance on Thursday's "Katie" with Katie Couric. "But age four, four to five, she stopped."



Since then, Brooke's height and weight of 16 pounds and 20 inches have remained a constant. She wears diapers, is pushed in a stroller, gets fed through a feeding tube due to a too-small esophagus, and communicates like an infant would, estimates her mother Melanie.



"Like 6 months," she explained on the show. "If she's happy, she'll giggle and laugh."



Doctors have told Melanie and Howard, who live in Maryland, that there is no other known case like Brooke's in the world. And at least one medical expert believes that Brooke's condition could hold the key to the fountain of youth.



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"Here's a woman 20 years old who has literally stopped aging," explained Dr. Eric Schadt, director of the Icahn Institute for Genomics and Multi-Scale Biology at Mt. Sinai Medical Center in New York, who also appeared on the broadcast. He said he took an interest in Brooke's case after years of her receiving no diagnosis from other doctors, and that her case could "blow a whole field of science wide open."



In the mean time, the parents provide care to her day in and day out, and have watched their younger daughter, 17-year-old Carly, eclipse her older sister by developing normally over the years. They admit they have often felt isolated in their struggle.



"It hasn't been easy," Melanie told Couric. "We don't have anybody really to turn to, to tell us what to expect." Doctors have no way to even determine how long Brooke may live.



Still, her parents say they take solace in the fact that she's here now, and believe she does not need to be cured of anything.



"If somebody knocked on the door right now and said, 'It's a guaranteed pill. Give this to Brooke and she'll be fixed,' well first I would say to him, 'She's not broken.' And B, I would say, 'Thank you, but no thank you," Howard said in a video filmed for the "Katie" show.



To research Brooke's condition, scientists are reading every single letter of the young woman's DNA. "That would fill, like, 3,000 of the fifth Harry Potter books that my kids like. That's how big her genome is," Dr. Schadt explained, adding that, so far, scientists have put her genes in fruit flies to see if it helps them lives longer. He added that the Greenberg's take solace in the fact Brooke has also helped with research into conditions including Alzheimer's, heart disease and cancer.



"That," he said, "is a pretty amazing gift to be able to give to humanity."



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