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."