Aug. 16, 2013 -- 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. Gabby 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 Gabby, as well as two other people who have striking similarities.

Why the 'Benjamin Button' children never age.

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 Gabby, 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 Gabby's 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."

16-year-old is the size of a toddler.

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 ABCNews.com. "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 ABCNews.com. "Other than that, she hasn't changed much since the [2012] show."

Williams, 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."

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."

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.

That is why he believes his study of Gabby Williams' genetic code is so important. "She fits the model," said Walker.

"We've been on this journey to find out, are my other children at any risk in having a child like Gabrielle," said Williams, who has five other children between the ages of 1 and 10.

"We did find out with Dr. Walker when he did the [gene] sequencing that it's not something we can pass on but just an abnormality, a mutated gene that was just happenstance," she said. "That was a relief for us."

At first, when the Williams family members found out about Walker's research, they hesitated to become guinea pigs in the studies that would promote a so-called "fountain of youth."

"There was some concern," she said. "We are good Catholics, God-fearing people and we believe we are meant to get old -- the process of life -- and meant to die. It was scary to think about, and we did not want to be part of it."

But as they talked further with Walker, the family realized that his research was designed to help people struggling with the impairments of old age.

"Alzheimer's is one of the scariest diseases out there," said Williams. "If what Gabrielle holds inside of her would find a cure -- for sure we would be a part of the research project. We have faith that Dr. Walker and the scientific community do find something focused more on the disease of aging, rather than making you 35 for the rest of your life."

As for Gabby's life span, her doctors cannot say what that will look like.

"From the time of her birth, we didn't think she would be with us very long," said her mother. "The fact is she is now going on 9 years. She kind of surpassed my expectations from the get go.

"It's not something I worry about," said Williams, who said she trusts that God has a plan for her infantile daughter.

"When he is ready to take her back, it will be sad," she said. "But what a glorious thing it will be for Gabby to go to heaven one day. I know it will happen, but I am not hoping it's any day soon."