Now, at age 16, the twins, who are from Maine, are brother and sister. Wyatt is now Nicole, undergoing transition with hormone treatment at Gender Management Service at Children's Hospital in Boston.
The majority of all children who express the belief that they are the wrong gender will enter puberty and go on to identify with their biological gender, according to endocrinologist Dr. Norman Spack, who treats Nicole.
"Of all little kids who are gender variant, 20 percent will stick with it," he told ABCNews.com when the Maines story was first reported in 2011. "Over 80 percent will accept looking like Jonas and would push talk about cutting off [their penis.]"
The Rev. Stan Sloan, CEO of Chicago House, which provides some of the nation's most comprehensive services for transgender people, said gender variant children need encouragement and support.
"Ultimately, they become the people they were meant to be," he said. "Maybe a boy wants to wear pink for a year and moves on, or he's in love with pink and transitions -- or doesn't transition."
Sloan, who is gay, said, "If I could have known as a little kid there was a camp where little kids were gay and struggling with the same thing, it would have made such a difference. ... If it does nothing else, it gives [kids] a few weeks in what feels like a world that is accepting and understanding and doesn't pass judgment."
So far, some of the camp's alumni, when they reached puberty, went on hormone blockers to begin transition from boy to girl. Others have decided they are gay, according to Snyder.
"There are definitely a variety of possibilities. For most parents, it's hard to believe that their kid will turn out straight and gender-conforming," Snyder said. "But statistically, it's true that some of our kids will end up being gender nonconforming, but still conforming enough to be straight."
Snyder said a psychologist who is treating his son has advised the parents to allow the boy to freely express himself. But PJ's parents also consulted a psychiatrist this year because of some social and behavioral issues.
The Snyders said they are open-minded and live in a diverse neighborhood where their three children play with children of same-sex couples. But other parents have a harder time accepting their gender variant children.
"What really riles people ... is they think that instantly you are forcing a child into being gay or lesbian or transsexual," he said. "It's not that at all. Our children really want to express a different gender identity and we have to be ready ... because you just don't know."
But for now, his father said, the "quickest way" his 10-year-old explains himself to others is: "I know I have long hair and wear girls' clothes, but I am just a boy who happens to like pink and purple."
"When I asked, 'Well, what is it that makes you a boy versus a girl?' he came up with a cool answer," said his father:
"I've got friends at school who really like soccer and wear t-shirts from professional soccer teams," PJ will say. "Just 'cause I wear a shirt doesn't make me a professional soccer player."
"I couldn't believe he said that," his father said.