Now that 8-year-old Gabby Williams has reached 11 pounds, she has graduated from newborn clothes to those meant for 3- to 6-month-olds. She has the features of a newborn; her mother diapers her and tends to her as if she were an infant.

Only a handful of people in the world like Gabby have a mysterious condition that drastically slows their rate of growth.

Figuring out exactly what is going on in these people is the topic of a TLC television special, "40-Year-Old Child: A New Case," which follows the initial story about Gabby, which aired last year.

Since the first special, Gabby hasn’t made much progress in the growth department.

"She’s gotten a few more haircuts," Gabby’s mother, Mary Margret Williams told ABCNews.com. "Other than that, she hasn't changed much since the [2012] show."

Medical researcher Richard F. Walker has been studying Gabby and two other people who share the condition, looking for the genetic off-switch which stops the aging process.

"In some people, something happens to them and the development process is retarded," Walker said. "The rate of change in the body slows and is negligible."

But unlike the seductive immortality flaunted by vampires and Greek gods, the condition leading to seeming immortality comes with a host of medical problems, including deafness and the inability to walk, eat or speak.

Walker says that physiological change — what he refers to as "developmental inertia" — is necessary for human growth. We begin to develop after birth, but maturation doesn’t stop.

"What happens is we become mature at age 20 and continue to change," he explains.

We start the first processes of aging in our 30s, and it just keeps going.

In one of the cases he is studying, Walker found damage to one of the genes that causes developmental inertia. The significance lies not in preserving a youthful body for frivolous means, but as a way to help battle the deleterious impairments associated with aging, like Alzheimer’s disease.

"If we could identify the gene and then at young adulthood we could silence the expression of developmental inertia, find an off-switch,” Walker says. “When you do that, there is perfect homeostasis and you are biologically immortal.”

As for Gabby's lifespan, no one can predict what her future will hold.

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

Watch scenes from "40-Year-Old Child: A New Case" below:

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