In all, she has undergone more than 150 surgeries, many to reconstruct or compensate for the missing internal organs.
She also has become a well-known personality in her home town of Kennebunkport, Maine, where she loves to go bowling with friends, including Samantha Mitchell and Zack Kellett, her cousins and frequent companions.
"All the bumps and nudges in her, in her path, she's just overcome them," said Kellett.
"All the kids love her," said Mitchell. "She's like their role model, basically."
Some kids get crushes on a teacher; Shiloh has one on her doctor. She calls him "Doctor Hottie."
Shiloh's mother works for an insurance company. Her father cares for her full-time, fixing her meals and assuring that her medications are mixed and ready. He also provides comic relief, with jokes and other antics, when she needs it.
"They're my parents," said Shiloh, "and I love them so much."
The family has a Web site, Shilohbenefit.com, which accepts donations to help cover the expenses of Shiloh's care and posts news of her progress.
"We've always understood that she's supposed to be here," said Leslie. "When nobody else has believed it, we believed it, because it's just the way she is."
It would be possible to surgically separate her legs, but not so soon after her kidney transplant. Shiloh says she doesn't want the surgery, anyway. "Dr. Matt asked me if I wanted to get my legs separated, and I said no. I said, better just deal with it and stick with the 'no' answer."
In fact, one of her favorite activities is dancing. She has taken ballet classes and participated in a rehearsal for a dance recital, sitting and swaying on the stage (she missed the recital itself because she fell ill). She is able to move remarkably well by scooting her body across the floor.
"She has every reason in the world to be bitter and unhappy and miserable," said her father. "And she's the exact opposite. She's happy. She's vibrant."
What future she has, no one can say, because the only other known survivors of Mermaid Syndrome are also still living. Of the two, one is younger than Shiloh: Milagros Cerron. She was the topic of worldwide headlines early in 2005 when doctors in Lima, Peru, successfully separated her legs nine months after her birth. She will face a future of surgeries to reconstruct her internal organs.
The other survivor is Tiffany Yorks, 19, who lives near Tampa, Fla.. As far as can be determined, no one ever has survived Mermaid Syndrome as long as she has.
ABC News arranged a meeting between Shiloh and Tiffany in Portland, because the syndrome is so rare neither of the girls had ever been able to talk with another like herself.
"There's so few survivors," said Tiffany. "I think it's very important for us to get close just because there's no one else to look up to. There's no one above us that knows what we have to deal with."
Early in her life, Tiffany had surgery to separate her legs and walked until she was nine, when she fell and injured a knee. She is now confined to a wheelchair. The partial kidney with which she was born failed two years ago, and she needs a transplant before she can go on with her life.
Since both girls had spent so much of their lives under hospital care, Shiloh's question to Tiffany was straight to the point. "Does it get any easier?"