Brooke Greenberg's 'Syndrome X' Condition Defies Age, Baffles Doctors (VIDEO)

WATCH: 20-Year-Old Toddler Hasn't Grown Up

Brooke Greenberg may appear to be no older than 4 or 5, but the girl, who suffers from a mysterious medical disorder, is actually 20 years old. Her extremely rare condition, dubbed "Syndrome X," has puzzled medical professionals for years. Even some of the world's leading experts have been unable to find a medical precedent for her stunted development. However, one doctor believes that studying Brooke's biology could offer clues to other mysteries of the human body.

Brooke progressed through life like any other young girl, until her parents noticed she stopped developing -- both physically and mentally -- around age 5.

"She has been the same weight, the same height for now 15 years," her father Howard said during a Jan. 10 segment on Katie Couric's daytime talkshow, "Katie." The only things that have continued to grow normally are Brooke's nails and hair, he also revealed.

Brooke's doctor and Johns Hopkins School of Medicine pediatrician Lawrence Pakula spoke with ABC's 20/20 in 2009. "Many of the best-known names in medicine, in their experience... had not seen anyone who matched up to Brooke," Pakula said. "She is always a surprise."

Eric Schadt, director of the Icahn Institute for Genomics and Multi-Scale Biology at Mount Sinai Medical Center in New York, says sequencing Brooke's DNA could be the key to learning more about human aging and longevity.

"It's an amazing phenotype. So here's a woman, 20 years old, who has literally stopped aging," Schadt said on "Katie" on Jan. 10. "From a scientific standpoint, cases like Brooke's can blow a whole field of science wide open by elucidating what's going on in her."

Working to pinpoint the aging process at a molecular level, Schadt and his team have inserted Brooke's genes into fruit flies to analyze how her genetic code affects the organism.

According to Schadt, manipulating Brooke's genes may not only increase longevity, but could also help treat age-related disorders, the International Business Times reports.

Schadt's work stems from earlier research involving Brooke's DNA that sought to identify the master genes that control the developmental process, according to Discovery News.

Although she is 20 years old, Brooke acts very much like a toddler and must be constantly looked after by her parents. Though it's been a struggle, Brooke's family says they have tried to make her life as normal as possible.

"It hasn't been easy, but each day you get better at doing your job," Brooke's mother, Melanie, told Couric.

Take a look at the video (above) to see a clip from the "Katie" segment on Brooke's condition. Visit the "Katie" website to view more clips from the segment.

Before You Go