Brooke Greenberg is the size of an infant, with the mental capacity of a toddler.
She turned 16 in January.
"Why doesn't she age?" Howard Greenberg, 52, asked of his daughter. "Is she the fountain of youth?"
Such questions are why scientists are fascinated by Brooke. Among the many documented instances of children who fail to grow or develop in some way, Brooke's case may be unique, according to her doctor, Johns Hopkins School of Medicine pediatrician Lawrence Pakula, in Baltimore.
CLICK HERE to see photos of Brooke through the years.
"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."
Brooke hasn't aged in the conventional sense. Dr. Richard Walker of the University of South Florida College of Medicine, in Tampa, says Brooke's body is not developing as a coordinated unit, but as independent parts that are out of sync. She has never been diagnosed with any known genetic syndrome or chromosomal abnormality that would help explain why.
In a recent paper for the journal "Mechanisms of Ageing and Development," Walker and his co-authors, who include Pakula and All Children's Hospital (St. Petersburg, Fla.) geneticist Maxine Sutcliffe chronicled a baffling range of inconsistencies in Brooke's aging process. She still has baby teeth at 16, for instance. And her bone age is estimated to be more like 10 years old.
"There've been very minimal changes in Brooke's brain," Walker said. "Various parts of her body, rather than all being at the same stage, seem to be disconnected."
Brooke's mother, Melanie Greenberg, 48, sees a different picture. "She loves to shop," Greenberg said. "Just like a woman."
Brooke rides in a stroller while her mom shops for clothes in the infant sections of department stores near their home in a Baltimore suburb. That Brooke is in her mid-teens is so mind-boggling that if another mother with a toddler asks Greenberg how old Brooke is, she usually doesn't try to explain.
"My system always has been to turn years into months," Greenberg said. "So, if someone asked today, I might say, she's 16 months old."
For more of Brooke's story, watch the documentary, "Child Frozen In Time," Sunday, Aug. 9 at 9 p.m. on TLC.
Brooke weighs 16 pounds and is 30 inches tall. She doesn't speak, but she laughs when she is happy, and she clearly recognizes the people around her. She has three sisters: Emily, 22; Caitlin, 19; and Carly, 13. All three are bright, active and of normal size and development. They say that Brooke has ways of expressing herself like the teenager she is.
"She looks like a 6-month-old, but she kind of has a personality of a 16-year-old," Caitlin said. "Sometimes we joke about how she rebels."
Brooke will resist and refuse activities that don't appeal to her by vocalizing her displeasure, not with words, but with sounds typical of an infant. "She makes it known what she likes and what she doesn't like," sister Emily said.
Carly said it no longer seems strange to have an older sister who is still essentially an infant. "As I got older, she was just like another little sister to me," she said.
In her first six years, Brooke went through a series of medical emergencies from which she recovered, often without explanation. She survived surgery for seven perforated stomach ulcers. She suffered a brain seizure followed by what was diagnosed as a stroke that weeks later left no apparent damage.
At 4, she fell into a lethargy that caused her to sleep for 14 days. Then, doctors diagnosed a brain tumor, and the Greenbergs bought a casket for her.
"We were preparing for our child to die," Howard Greenberg said. "We were saying goodbye. And, then, we got a call that there was some change; that Brooke had opened her eyes and she was fine. There was no tumor. She overcomes every obstacle that is thrown her way."
Brooke's doctor said the source of her sudden illnesses remains a mystery.
"We often did not have a good explanation for why she became ill as quickly and intensely as she did," Pakula said. "There were many times in which there were real doubts about her ability to survive."
As she rocks back and forth in a baby swing, Brooke is fed through a tube inserted into her stomach, because her esophagus is so small that swallowed food could back up into her lungs and cause pneumonia.
Doctors recommended growth hormone therapy early in Brooke's life, but the treatment produced no results.
Howard Greenberg recalled the follow-up visit to the endocrinologist. "We took her back in six months, and the doctor looked at us and said, 'Why didn't you give Brooke the growth hormones?' And I said, 'We gave Brooke the growth hormones. We gave her everything you told us to do.' And Brooke didn't put on a pound, an ounce; she didn't grow an inch."
Brooke's hair and her nails are the only two things that grow, Howard said. "She has pajamas and outfits that are 10 or 12 years old," he said.
One of the things she loves most is movement. As Brooke lies on her stomach, Carly often steers her through the house on an ottoman. Brooke also likes to push against open kitchen drawers until they slam shut.
In her crib, "she's very content," Howard said. "She has very little conception of time."
The family has placed a small television near the crib so she can watch whenever she pleases. Her father gets up in the middle of each night to check on her.
Brooke has a caretaker during daytime hours, but the family's schedule revolves around her, year after year. The Greenbergs take no vacations, have few nights out and involve Brooke in as many family activities as possible.
"To go to a swimming pool for the summer, or belong to a summer club ... we tried all those things, and it's lacking something," her mother said. "Brooke's not there. We're not a family without Brooke."
Brooke goes to a Baltimore County public school, Ridge Ruxton, dedicated to special education. Based on her age, she would be a junior in high school. Jewel Adiele, one of Brooke's teachers, said she wonders sometimes what Brooke is thinking or perceiving.
"People who have worked with her in the past or who briefly see her say ... there's no change," Adiele said. "But I think, in her heart, she changes. I think from day to day, there are changes. They're not just as visible as you see in a lot of teens."
To try to determine why Brooke's aging process has been so irregular -- and what it means to the understanding of our genetic makeup -- Walker and Sutcliffe have studied samples of Brooke's cells and DNA to look for what they think may be a genetic mutation never seen before that has affected the way she ages.
Walker, of the University of South Florida, believes that if the gene can be isolated, it may provide clues to questions about why we age and die.
"Without being sensational, I'd say this is an opportunity for us to answer the question, why we're mortal, or at least to test it," Walker said. "And if we're wrong, we can discard it. But if we're right, we've got the golden ring."
If the gene -- or complex of genes -- is identified, Walker plans to test laboratory animals to determine whether the gene can be switched off and, if so, whether it will cause the animal's aging to slow.
In the long term, the idea that the aging process might somehow be manipulated raises serious questions about what human beings might do with that knowledge.
"Clearly, that's the science fiction aspect of it," said Walker, describing the social and ethical dilemmas that would arise. "We can't have continued reproduction and people who don't age."
One possible reason to slow the aging process, Walker suggested, would be to allow astronauts to travel in space for long periods of time. "But right now, it's only conjecture," he said.
Neither Walker nor Pakula, her doctor, can speculate how long Brooke's life might be. "That's more of a crystal ball question," Pakula said. "I think there's no way of knowing. "
The visual evidence of that unpredictable future is always there in the family pictures -- photographs in which everyone but Brooke is aging.
The Greenbergs are fascinated by the promise that a scientific breakthrough may stem from Brooke, whose own life is governed by the most basic elements: food and shelter; a family's love; and their ability to see in her far more than meets the eye, having come to terms with the prospect that she will never grow up.
"We love her just the way she is," Melanie Greenberg said. "We don't want to change her."
Added Howard Greenberg, "Brooke is the nucleus of our family. What if Brooke holds the secret to aging? We'd like to find out. We'd like to help people. Everybody's here for a reason. Maybe this is why Brooke is here."
For more of Brooke's story, watch the documentary, "Child Frozen In Time," Sunday, Aug. 9 at 10 p.m. on TLC.