When we first met Lindsay Ratcliffe, she was just like any other first grader, who loved ponies, Legos and running the bases at the T-Ball game in her hometown of Flat Rock, Mich.
But at 20 pounds and 36 inches, she was not an ordinary six-year-old. Lindsay has a rare and fatal disease called progeria -- derived from the Greek word for "prematurely old" -- which makes her body age eight to 10 times faster than normal children.
Only 80 children in the world currently have the condition, including 18 in the United States, according to the Progeria Research Foundation. Children are born seemingly healthy, but start aging dramatically by the age of 2. On average, they die at 13.
"You see this vibrant 5-year-old and then you put her X-Rays up and it looks like someone of a senior age," said Dr. Heidi Labo, who is Lindsay's chiropractor and aunt.
At birth, Lindsay showed no signs of progeria. "The first thing I did, I counted her fingers. I counted her toes. I'm like, 'Ten fingers, ten toes, we're good,'" recalled her father, Joe Ratcliffe, who had just returned from serving in Iraq as a U.S. army cook.
After four months, Lindsay had gained little weight and her parents knew something was seriously wrong. Specialists ran tests for weeks for nearly every disease and syndrome before reaching the terrifying diagnosis of progeria. It was a disease the family had never heard of.
"What scared me the most was they told us, 'We don't know [much about progeria],'" said Joe Ratcliffe. "'You can go to a website and that is best source of information. You're going to have to teach the doctors what to do.'"
The Ratcliffes soon learned that progeria was the rarest of rare diseases, affecting only one in every 4-to-8 million births. It is caused by a mutation in a gene called LMNA, but is not hereditary.
"Kids with progeria occur out of blue. There's no family history, no warning, no reason to think that this might be getting ready to happen," said Dr. Francis Collins, the scientist who first discovered the gene and is now the director of the National Institutes of Health.
At the time of Lindsay's diagnosis, there was no known treatment for progeria and no cure, leaving the Ratcliffes to care for their beloved baby, whom they knew they would lose too soon.
"In the beginning it was a lot harder because Lindsay couldn't talk, she couldn't walk and she couldn't do everything for herself," Kristy told ABC's Barbara Walters. "Now, it's so easy because you see her and you smile. You can't help it."
They are determined to make every moment count. "Whether it's a birthday or going to the park, whatever you do, you look at it as unfortunately it could be the last time," her father said.
Young Girl Lives in Body Older Than Her Grandmother's
Being a young girl in a body that is biologically older than that of her grandmother takes a toll. Lindsay's leg muscles hurt at school during recess, so every week she goes to the chiropractor for a checkup and adjustment.
"She's six years old and she runs and jumps but has the spine of 70 year-old," Labo told us last year. "She feels aches and pains and isn't aware of what it is, but there is arthritis forming slowly throughout the spine."
For most of her life, Lindsay has been protected within a cocoon of love and empathy. But sometimes, the strangeness of her symptoms -- especially her tiny size -- strikes a brutal blow to her self-esteem. When the spunky little girl is mistaken for a two-year old, she instantly deflates and responds: "I'm not a baby."
"That'll of change the whole mood of her," Joe said. "You see her face and you see that for a brief moment the glow has gone out her eye."
Her mother tries to shield Lindsay from the constant stares. "I try to position myself so that she doesn't see it because I don't want it to hurt her as much as it hurts me," Kristy Ratcliffe said.
Three-Feet Tall and Fearless
As one of only nine girls in the U.S. who has progeria, it can be isolating for Lindsay. Amazingly, Kaylee Halko, now 8, who has the same rare condition, lives an hour away in Monclova, Ohio. The two girls have become friends and because of progeria look strikingly similar. Kaylee is a confident, boisterous and fearless.
"[Kaylee] likes to say she's a star," her mother Marla Halko said.
The youngest of four children in the Halko brood, Kaylee -- at only three feet tall and weighing 24 pounds -- is noticeably different from her older brothers, but they have a close, loving relationship.
"We don't really think about her having a disease. We just think of her as a normal person," her eldest brother T.J. said.
Kaylee loves to dance and is enrolled in a cheer dance class with regular kids. Despite worries about osteoporosis, a common symptom of progeria which makes her bones unusually brittle, Kaylee insists on riding the big yellow bus to school just like her brothers and millions of other kids across the country.
The pint-sized chatterbox told Walters that the main difference between them was their hair. "I have a bald head and you have hair," said Kaylee, who longs to grow curly hair and sometimes wears wigs.
"She's so happy and she has this condition and she just lives!" said her mother.
Finding a Cure for Progeria
All this optimism may seem oddly out of place, when Kaylee is unlikely to live to be a teenager. The life expectancy of most children with progeria is only 13. She is participating in one of two clinical drug trials aimed at developing a cure for progeria and takes several different medicines with the hope of slowing down the aging process.
"It is too early to tell if it working or not," said Marla Halko.
Lindsay is also enrolled in the drug trial along with 27 other children. She completed a part of this experimental program in 2009. Now, she and Kaylee have joined a second larger trial which includes 45 progeria patients from 24 different countries.
Another one of the participants is Hayley Okines of Great Britain; at 13-years-old she is one of the oldest surviving children with progeria in the world. Like Lindsay and Kaylee, she has the hallmark bald head, tiny stature and lives her life in the shadow of an ever-present threat.
"The first thing I think about when I wake up in the morning is whether today is going to be the day I lose her," said her mother Kerry Okines. "The only way I cope is by saying to myself that Hayley's going to be the one to prove the doctors all wrong."
Hayley and her parents have been hopeful ever since she joined the progeria drug trial, which is testing a pill originally developed for cancer, which could reverse the dramatic instability in her cells. She believes she sees small signs the pill is working.
"I've got eyebrows and I've got eyelashes and I've got hair on my arms," she said.
Despite progeria, Hayley sees herself as a person not a disease and has all the interests of an average pre-teen, including fashion, photography and a crush on pop-star Justin Bieber.
Parents Hope Not to Outlive Children
The Okines, the Ratcliffes and the Halkos hope that the ongoing clinic trials and research will unlock the secrets to their daughters' medical mystery and the process of normal aging.
"The evidence is growing, that the same glitch these kids have that causes them to produce a protein that makes their cells unable to keep dividing, that same protein is being produced by all of us," said Collins. "It looks as if this maybe part of a program that prevents humans from being immortal."
Every year, the three families raise money for the progeria research, typically through walk-a-thons.
Kaylee is very aware of the high cost of research and participating in clinic trials. She told Walters her wish is to, "that I have thousands of dollars to buy my medicines."
The families all say there is no preparing yourself for the probability that you will outlive your child.
"It scares me. Not much scares me, but that does. Even thinking about it [scares me], so I generally don't allow myself to go there," said Joe Ratcliffe, who tattooed the Progeria Research Foundation's logo of a child's handprint and dove on his arm as a symbol of hope. "That's what we fight for. Without awareness we have nothing."
"20/20" first met the girls last year and caught up with them again this week. They continue to be determined to enjoy life, however short it may be. For Lindsay, her first trip to the beach was a brave new world of discovery.
"She saw the ocean and was just in awe of all this water," recalled her father, Joe.
Lindsay is usually unable to stay in the water for more than ten minutes. Due to progeria, she has very little body fat and cannot tolerate the cold. But her parents came up with an ingenious solution -- a wet suit -- specially tailored for her small size. Lindsay was ecstatic when she saw it.
"Making a big deal out of things that are big to her is just going to make her happier," said Kristy. "We just want to make sure she has as much happiness as she can hold."
This summer, Lindsay, now 7, returned to the beach, swimming freely thanks to her wetsuit.
Hayley is enjoying school and despite her size has many friends both in class and around the world. Recently, her dream came true, when she met her idol Justin Bieber thanks to a Twitter campaign.
Kaylee, too, is enjoying public school and is preparing to enter the second grade in the fall. Her mother Marla was afraid of her riding bus alone but not Kaylee, with her enormous backpack on her tiny frame, she took one small step towards education and one huge leap into her future.
She is now taking hip-hop classes and dreams of starring in her own television show.
Kaylee has advice for all the children who share her disease: "Be fabulous. Don't let one thing ruin your life!"