That's when the abuse began, she says.
"I was abused, mentally, physically, everything," she says in Cosmopolitan's February issue, on newsstands Jan. 8.
While attending the prestigious Italia Conti Academy of Arts, she says the adults around her took advantage of her being without parental supervision: "I was 10 years old looking like I was 28, being a very sensual dancer."
"I was a tormented little kid who had to put on this sexy façade because that was my job and my life," she adds. "But my heart was the same, and I was this innocent little girl. I wanted so much love."
The actress, now 24, says the abuse became worse "when I started hitting puberty, when I started becoming a woman and stopped being a little girl."
Her classmates included future Dancing with the Stars pros Mark Ballas and Julianne's brother Derek Hough, both also children at the time. They had formed their own pop music group when Julianne was 12 performing in the U.S. and the U.K.
Hough declines to say who abused her or provide other details.
"I'm a very forgiving person, and I don't want to hurt anybody," she says. "What's past is past."
But the painful emotions surfaced during filming of her latest movie, Safe Haven, opening Feb. 14, about a girl who survives an abusive ex-boyfriend and accepts the love of a widower, played by Josh Duhamel.
During one particularly difficult scene, she says, "I went from bawling to containing to laughing to crying again. Josh was crying. I think it was the most therapeutic moment of my life."
As for her own love life, Hough says that she and boyfriend Ryan Seacrest work to find time for each other around their busy schedules.
"We love what we do," she says. "We take pride in giving it our all, but then when we're alone, we really focus in on going to dinners and being extra-romantic and affectionate and just being there for each other."