By all appearances Brooke Greenberg was a toddler. But the Reisterstown, Md., native who had a rare genetic condition was in fact 20 years old.

Greenberg, who was diagnosed with a condition that affects just six people in the world, died on Oct. 24. With the condition known as “Syndrome X,” Greenberg remained roughly the size of a 6-month-old child and showed no signs of any genetic abnormalities.

“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, who delivered the eulogy at her funeral, 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.”

At birth, one month before her due date, Greenberg weighed just 4 pounds and had a rare condition called anterior hip dislocation.

"Her hips were dislocated from where they normally would be," Dr. Lawrence Pakula, Brooke's pediatrician, told NBC News. "Hers were pushing forward and put her legs in a very awkward position."

Her parents realized something was wrong developmentally when Brooke’s younger sister began to outgrow her. "She really was not growing to the rate that she should have been growing," her father, Howard, said. "I remember the doctor saying that when Brooke enters, say, 4th grade, she’ll be like at the first-grade level. We could live with that, we had no problem with that. We really did not know the road we were about to take with Brooke."

Even after Brooke had been brought to numerous specialists, her condition continued to stump doctors.

"They [the specialists] just said she’ll catch up. Then we went to the nutritionist, the endocrinologist. We tried the growth hormone," her mother Melanie said, adding there was no change in Brooke’s growth.

"I mean she did not put on an ounce or she did not grow an inch," Howard Greenberg said. "That’s when I knew there was a problem."

Throughout her life, Brooke began to develop serious medical conditions including stomach problems that required her to be fed through a feeding pump. She was also frequently hospitalized for respiratory problems and suffered a stroke. At age 4, Brooke became lethargic and slept for 14 days. At the time, doctors diagnosed her with a brain tumor.

“We were preparing for our child to die," Howard Greenberg told ABC News. "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."

Throughout her medical emergencies, doctors remained clueless about the causes of her rare illness.

"We often did not have a good explanation for why she became ill as quickly and intensely as she did," Johns Hopkins School of Medicine pediatrician Lawrence Pakula said.

Despite her size and medical challenges, Brooke’s mannerisms changed as she aged.

"She looks like a 6-month-old, but she kind of has a personality of a 16-year-old," her sister Caitlin told ABC News in 2009. "Sometimes we joke about how she rebels."

Brooke did not speak, but used sounds to express her content or displeasure. She giggled when she was happy and made sounds, similar to an infant, to refuse an activity. "She makes it known what she likes and what she doesn't like," her sister Emily said.

Doctors studied Brooke’s mysterious condition throughout her life. Recently, her parents have consented to having Brooke’s DNA sequenced by researchers at the Icahn Institute for Genomics and Multi-Scale Biology at Mount Sinai Medical Center in New York to shed light on her condition.

"Brooke is the nucleus of our family,” Howard Greenberg said. “What if Brooke holds the secret to aging? We'd like to find out. We'd like to help people. Everybody's here for a reason. Maybe this is why Brooke is here."