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When Jacob Lemay invites you into his bedroom, he wants to give the full tour. He’ll show you his books, the planets and stars on the wall, his bunk bed and the stuffed animals he’s lined up all in a row.

“This one is Chase, and that one is Zizzy,” he says in the voice of a confident five-year-old. “And that one is Biscuit, and that one is Zarzo, and that one is Cheety, and that one is Snowflake, and that one is Fuzz."

Jacob is a happy, healthy, well-adjusted pre-schooler. He has two sisters — one older and one younger.

But it wasn’t always this way. In the beginning, his parents Mimi and Joe were raising three daughters.

WATCH: NBC's Kate Snow report on Jacob's story

When Jacob was born, his name was Mia. But by the time he was two, he was telling his parents, “I’m a boy.”

Last year, when he was four, they made a decision: to let him live as he has always identified — as a boy.

Jacob plays as an infant when he was still called Mia. Courtesy of the Lemay family

Some may think that's too young to make such a change, but many doctors who specialize in working with transgender children believe it's right for certain kids — those who show a rock-solid and enduring belief in their gender identity.

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“When kids are consistent, persistent, and insistent in a cross-gender identity — and wanting to be the other gender and wanting the other gender's body parts or being very unhappy with the body parts they're given — we consider those children very likely to go on and continue a transgender identity,” said Dr. Michelle Forcier, an associate professor of pediatrics at Brown University School of Medicine.

For those children, she and other pediatricians say, it can often be better to make a change sooner.

“The biggest harm is to not do anything," said Forcier, who is not Jacob's doctor but has a specialty in treating transgender children.

When a kid is told, ‘I don't see you,’ ‘I don't hear you,’ ‘I don't love you just the way you are,’ that's a pretty powerful message about conditional love.”

Ask Jacob why he transitioned and his answer is simple and straight-forward: "I wanted to be a boy."

Jacob dresses as Prince Charming at Disneyland with his sister Ella as a princess. Courtesy of the Lemay family

His parents say they decided to share Jacob's story because they can't afford not to.

“The environment that my son will grow up in depends on how transgender people are perceived by society,” Mimi says.

“He's gonna go out there in the world. He's gonna travel and he's gonna meet other people. If we don't come out now and talk to people and… show people that transgender children are normal and wonderful and they're not to be feared, then I'm afraid that he will go into the world and meet with hostility," she said.

“I can do my piece right now and I believe that my piece is speaking up.”

Mimi says that before the transition, her middle child would poke a body he couldn't embrace, saying things like “Why did God make me this way?” or “Why did God make me wrong?”

Mimi and Joe were scared and confused by the behavior and comments. But after seeing doctors and scouring the Internet for information — and after seeing how happy he was to dress as Prince Charming on a trip to Disneyworld — they decided last summer to make the transition.

They cut Jacob’s hair shorter, changed his wardrobe, and asked family and friends to refer to him as Jacob and use the pronouns “him” and “he.” Jacob started at a new pre-school last fall where none of the children knew him as Mia.

Jacob before and after having his hair cut. Courtesy of the Lemay family

For young children, there is no surgery or hormonal therapy. At this stage, before puberty, transgender children are making a more cosmetic change, Forcier says.

“We let them be themselves. So, they cut their hair and they wear their clothes and they wear their shoes they want. And they wear jewelry or they play with the kids they want to play with and they do the activities they want to do," she says. "We call that social transition.”

The Lemays understand that some will have a hard time with their decision. But they say they are convinced they did the right thing for their son, and Mimi recently wrote a letter to Jacob detailing their thoughts and feelings about this journey.

“Ultimately Jacob has made that choice in his mind and his heart,” Mimi says. “It's whether or not we accept it or not.”