From PEOPLE Magazine Click to enlarge
As soon as Brooke Greenberg enters her summer-school classroom, she starts eyeing the door to leave. Her noisy classmates irritate her. She pays little attention to her teacher. "She loves to be on her own, away from everybody," says her mother, Melanie.

Her behavior almost makes her sound like the teenager she officially is, but Brooke is like no other 16-year-old in the world. When she roams the hallways, she scoots in a pint-size walker; when she watches her favorite show, SpongeBob SquarePants, she does so from a crib. Born with a still unidentified syndrome that has doctors completely baffled, Brooke looks and acts like a 6-month-old. Weighing just 16 lbs. and standing 2'6" (a toddler's height), Brooke doesn't grow, except for her hair and nails. She is little different from the fragile baby her mom sang lullabies to back in 1993. "I've been focused on her for three years," says Dr. Richard Walker, a neuroendocrinologist studying Brooke. "I've yet to find anyone comparable."

Brooke came to the world's attention eight years ago, when news organizations, including PEOPLE (Sept. 17, 2001), shared the story of this seemingly ageless child and her family in suburban Baltimore. Now the focus of a TLC documentary (Child Frozen in Time, Aug. 9, 9 p.m. ET), Brooke has lost baby fat and learned to walk with support, says Melanie, but otherwise "it's like the movie Groundhog Day—the same every day."

For years Brooke's routine consisted of doctor visits and hospital stays. Delivered early by C-section (the baby's slow growth concerned Melanie's obstetrician), she weighed just 4 lbs. 1 oz. at birth and had severely dislocated hips. As the months passed, Brooke's development lagged and a list of medical issues grew. By Melanie's count, she's seen 50 to 100 specialists, though none have come close to solving the mystery of her condition.

Brooke's story fascinated Dr. Walker of Tampa, whose tests have led him to conclude that Brooke's body is "not developing as a unit." Mentally, she's like an infant: She giggles and cries, lights up when she sees her mom and reaches out to be held; she also still wears diapers and has never spoken a word. Her dental age is that of an 8-year-old (she has a mix of baby and permanent teeth) and her cellular age is "comparable to a 16-year-old's," though she hasn't gone through puberty and likely never will, Walker says. She's already suffered a lifetime's worth of illnesses: pneumonia, seizures, a brain mass and, at 4, a coma-like sleep that lasted 14 days and prompted her terrified parents to pick out a coffin. "I still feel," says Howard, 52, "when I go to check on her, she'll be gone."

No one has felt the impact of Brooke's condition more than Melanie. While Howard, a marketing associate for Sysco foods, worked and the couple's three healthy daughters—Carly, now 13, Caitlin, 19, and Emily, 22—attended school, Melanie was caring for Brooke. In between feedings (Brooke can't swallow normally, so she receives a special formula through a tube in her stomach), "I'd rock her and sing to her," Melanie, 48, recalls. "I have a bad neck because I held her for 12 years."

The physical, emotional and financial burdens (the Greenbergs refinanced their home at least twice to pay medical costs) eased after Brooke qualified for state-supported nursing care four years ago. On weekdays a nurse accompanies her to Ridge Ruxton, a special-needs school. At home, in her bright yellow room filled with Beanie Babies, she loves to glide in her infant swing. She adores her sisters, laughing with delight when they push her down the hall on an ottoman. "She's fun-loving," Carly says. Caitlin, a sophomore at Towson University, says if anything were to happen to her parents, she'd take over some of Brooke's care without hesitation. "She's my sister," Caitlin says. "I love her and I wouldn't change her for anything."

Nor would her parents, even as they've struggled to balance Brooke's needs as well as those of her sisters. "I made sure they went to sleepaway camp," says Melanie, "to get away and have a normal life." Melanie has regained some normalcy too, having returned to work as a graphic designer. Still, it's painful to hear friends speak of their daughters' proms and college plans. "It's sad," she says. "That should be Brooke."

No one knows how long Brooke will live. Doctors initially told the Greenbergs she wouldn't make it to 5. Now Howard believes his daughter will outlive him and that she'll help medical researchers unravel the mysteries of aging. "She's here," he says, "for a reason."

Melanie long ago stopped looking for answers. She just wants her daughter to have as normal and joyful a life as she can. After Brooke's 13th birthday, relatives gathered in the basement of the Greenberg home to celebrate her bat mitzvah; Brooke's sisters and her cousin read the prayers for her. In a pink dress and patent leather shoes, Brooke smiled and cooed as her adoring family passed her around. "The best we can do," Melanie says, "is make her happy."