They live half a world apart, but Gabby Williams and Nick Freeman share an incredibly rare and tragic bond - they are "Benjamin Button children".
Only about a half dozen sufferers exist in the world - they age only one year for every four in the life of a normal human being. And medical tests can find nothing wrong with them.
Their families met when Nick - who lives in Western Australia - travelled to the United States to consult with Dr Richard Walker of University of South Florida College of Medicine, who is studying their genetic codes to try and provide answers on how the condition is caused.
Dr Walker says their bodies do not develop as a coordinated unit, but as independent parts that are out of sync. But no known genetic syndromes or chromosomal abnormalities can explain why.
"It was great," said Gabby's mum, Mary, of meeting Nick and his mum Kaylee. "I think Nicky has a lot more abilities than Gabby. He walks and I don't know if she will get to that point. But talking with his mother, there was a connection for me."
Nick and Gabby also starred together in a documentary in the United States called "My 40-year-old Child" in 2011.
Nick is 42 years old but is trapped in a 10-year-old's body. He was born blind and struggled to survive at birth. When he was 1, he said his first words: "Mummy," "Daddy" and "Teddy." But at 14 months old, he stopped talking and hasn't spoken since.
His mother, Kayleen Freeman, told the documentary makers that she was convinced her son understands things, because he will pick up conversations and "burst into laughter." He loves music, "loud and fast" and makes himself understood with his body language. He also has a huge appetite, eating twice what an adult eats.
"He really loves having a massage," Kaylee says. "He comes over to me and shows exactly where he wants his back rubbed."
"He is a fighter and he is meant to be here. He loves horse riding and in the season goes up to three times a week. He loves travel, especially long car trips."