(NEW YORK) -- Gabby Williams has the facial features and skin of a newborn, and she is just as dependent. Her mother feeds, diapers and cradles her tiny frame as she did the day she was born.
The little girl from Billings, Mont., is 8 years old, but weighs only 11 pounds. Williams has a mysterious condition, shared by only a handful of others in the world, that slows her rate of aging.
For the past two years, a doctor who has been trying to find the genetic off-switch to stop the aging process has been studying Williams, as well as two other people who have striking similarities.
A 29-year-old Florida man has the body of a 10-year-old, and a 31-year-old Brazilian woman is the size of a 2-year-old. Like Williams, neither seems to grow older.
Unraveling what these three people may have in common is the subject of a TLC television special, 40-Year-Old Child: A New Case, which airs Monday, Aug. 19, at 10 p.m. ET. The show is a follow-up to Williams' story, which aired last year.
"In some people, something happens to them and the development process is retarded," said medical researcher Richard F. Walker. "The rate of change in the body slows and is negligible."
Walker is retired from the University of Florida Medical School and now does his research at All Children's Hospital in St. Petersburg.
"My whole career has been focused on the aging process," he told ABC News. "My fixation has been not on the consequences but the cause of it."
Not only do the people he's studying have a growth rate of one-fifth the speed of others, but they live with a variety of other medical problems, including deafness, the inability to walk, eat or even speak.
"Gabrielle hasn't changed since pretty much forever," said her mother, Mary Margret Williams, 38. "She has gotten a little longer and we have jumped into putting her in size 3-6 month clothes instead of 0-3 months for the footies."
"Last time we weighed her she was up a pound to 11 pounds and she's gotten a few more haircuts," she told ABC News. "Other than that, she hasn't changed much since the  show."
Mary, who works part-time at a dermatologist's office, and her husband, a corrections officer for the state, share the child care responsibilities for their perpetual infant.
Walker explains that physiological change, or what he calls "developmental inertia," is essential for human growth. Maturation occurs after reproduction.
"Without that process we never develop," he said. "When we develop, all the pieces of our body come together and change and are coordinated. Otherwise, there would be chaos."
But, said Walker, the body does not have a "stop switch" for this development.
"What happens is we become mature at age 20 and continue to change," he said.
The first subtle internal body changes of aging are seen in the 30s and become more visible in the 40s.
"There is a progressive erosion of internal order as a result of developmental inertia," he said.
In one of the girls Walker has studied, he found damage to one of the genes that causes developmental inertia, a finding that he said is significant. He also suspects the mutations are on the regulatory genes on the second female X chromosome.
"If we could identify the gene and then at young adulthood we could silence the expression of developmental inertia, find an off-switch, when you do that, there is perfect homeostasis and you are biologically immortal," he said.
Now, Walker doesn't mean that people will never die. Disease and accidents will still end human life.
"But you wouldn't have the later years -- you'd remain physically and functionally able," he said.
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