Maryland 20-Year-Old Dies Never Having Aged Brooke Greenberg is only one of a dozen children with syndrome X.

Oct. 29, 2013 -- Brooke Greenberg, who baffled scientists because she never aged, has died at the age of 20, never having developed beyond the physical size of an infant or the mental capacity of a 2-year-old.

The daughter of Howard and Melanie Greenberg from Reisterstown, Md., Brooke is one of about a dozen children in the world who have what some call syndrome X -- a kind of Benjamin Button disorder that prevents them from aging.

Her funeral was Sunday at a synagogue outside Baltimore, family friends confirmed.

"The family is doing as well as can be expected," Chris Cole, a colleague of Brooke's father, told ABCNews.com today. "They are going through their traditions this week -- the shiva."

Doctors try to unravel the secrets of eternal youth.

Brooke has been pushed around in a stroller all her life. In 2009, when her family was interviewed on ABC's "20/20," Brook weighed 16 pounds and was 30 inches tall. She didn't speak, but she laughed when she was happy, and clearly recognized her three sisters: Emily, now 26; Caitlin, now 23; and Carly, now 17.

But only her hair and fingernails grew.

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 had a brain seizure followed by what was diagnosed as a stroke that, weeks later, had left no apparent damage.

At 4, she fell into a lethargy that caused her to sleep for 14 days. Then, doctors diagnosed a brain tumor, and the Greenbergs bought a casket for Brooke.

"We were preparing for our child to die," Howard Greenberg told ABC in 2009. "We were saying goodbye. And, then, we got a call that there was some change -- that Brooke had opened her eyes and she was fine. There was no tumor. She overcomes every obstacle that is thrown her way."

Richard F. Walker, a retired medical researcher from the University of Florida Medical School who now does his research at All Children's Hospital in St. Petersburg, has followed Brooke's case since she was about 2 years old, comparing her genetic code with that of other children with the same condition.

"In some people, something happens to them and the development process is retarded," he told ABCNews.com earlier this year. "The rate of change in the body slows and is negligible."

Walker wants to learn not only what is wrong with these children but also if others in the family could pass on genes for this rare and baffling disorder.

Their bodies do not develop as a coordinated unit, but as independent parts that are out of sync, according to Walker. No known genetic syndromes or chromosomal abnormalities can explain why.

Brooke still had baby teeth at age 16, and her bone age was estimated to be more like 10.

"There've been very minimal changes in Brooke's brain," he said. "Various parts of her body, rather than all being at the same stage, seem to be disconnected."

Walker also studies Gabby Williams, an 8-year-old from Billings, Mont., who weighs only 11 pounds and a 29-year-old Florida man who has the body of a 10-year-old. Like Brooke, they never seem to age.

"My whole career has been focused on the aging process," he said. "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.

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 called 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. Human life will still end.

"But you wouldn't have the later years -- you'd remain physically and functionally able," he said.

As for Brooke, even though she never grew up, her rabbi remembered her two-decade life with dignity and reverence in a eulogy this week.

"While the outside world may have noticed Brooke's physical stature and been puzzled by her unique development state, she brought joy and love to her family," Rabbi Andrew Busch told the New York Daily News. "Her parents, three sisters and extended family showered her with love and respected her dignity throughout her entire life."