Brooke Greenberg, the girl who never aged, has died (Facebook)

A woman dubbed the "fountain of youth" as she never aged beyond toddlerhood has died.

Brooke Greenberg died last week at the age of 20. The cause of death is unknown at present. She had puzzled doctors for years and eventually was diagnosed by Syndrome X.

Rabi Andrew Busch delivered the eulogy at her funeral. He told New York Daily News: "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.

"Her parents, three sisters and extended family showered her with love and respected her dignity throughout her entire life."

She was buried at the Baltimore Hebrew Centre at the weekend, three days after her death.

Greenberg looked and acted like a toddler throughout her entire life. Scientists studied her DNA to learn more about human aging and extending life.

Geneticists found her genes associated with premature aging diseases were normal, however. Most of her problems were a result of her body running out of sync. One of her doctors, Richard Walker explained in 2009: "Various parts of her body, rather than all being at the same stage, seem to be disconnected."

brooke greenberg
Greenberg was buried in Baltimore Hebrew Centre three days after her death (Facebook)

She was born prematurely on 8 January 1993 weighing just four pounds. She appeared normal at first but began going through unexplained medical emergencies, including stomach ulcers, seizures and a stroke.

Doctors also found a brain tumour when she slipped into a coma that disappeared after she woke. She stopped growing between the ages of four and five.

Walker said her brain was similar to that of a newborn while her bones were of a child aged 10.

According to New York Daily News, Greenberg would not develop for a month in the womb and then play catch up the next.

Doctors told the family to prepare for her death a number of times. They had picked out her casket and funeral clothes years ago.

Her family said she grew rebellious during her teens, developed a strong identity later in her life and loved her family.

Her father Howard Greenberg told New York Daily News: "She literally is the fountain of youth if you think about it. She's shown me that as hard as it gets sometimes, the next day it can only get better."