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.