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ABC News' Pierre Thomas and Jason Ryan report:

Olivia McNamara was starting her freshman year at Vanderbilt University when she applied for her first credit card.

"I was applying to get a student credit card the summer before I came to college, and I was denied from the first credit card company that I applied to," said McNamara.

After being rejected twice, she did some digging and found that someone had stolen her identity and had run up massive debt - to the tune of $1.5 million.

"I can't even describe it," she told ABC News. "It's just really shocking and we just had no idea."

What was even more shocking to McNamara and her family was that the crime had started when she was 9. Someone had stolen her Social Security number and set up false identities and more than 42 accounts. All of them had defaulted.

"They took loans out on boats and houses and everything," McNamara said.

Months after uncovering the crime, McNamara is still working to clear her credit. She still faces being rejected if she applies for a credit card or student loan or tries to rent a car or a house.

"I don't know why this happened to me," she said. "It's been very frustrating, very difficult for us to fix, and we don't really know how this is gonna affect me in the future."

McNamara is not alone in her ordeal. Thieves took 11-year-old Brianna's Social Security number and ran up more than $132,000 in debt, buying a car and a house.

Using 8-year-old Bradley's identity, thieves took out two student loans and got five credit cards. The damage totaled $19,200.

Authorities say that children are increasingly becoming the preferred target of identity thieves.

"We've seen children have this crime begin as early as 5 months old and then it goes on for years," said Bo Holland, founder and CEO of All Clear ID, a company that offers basic identity theft protection to consumers.

"A parent will typically find out when their child is moving into adulthood," Holland added. "When they are about to go to college, they apply for that first loan and, boom, they get denied."

In the last three years, there have been 57,000 cases of child identity theft reported to the Federal Trade Commission. A new report from All Clear ID estimates that one in 10 U.S. children are victims.

Criminals can hack home computers in search of tax forms with a child's Social Security number. They also can target hospitals, child-welfare agencies and even schools.

"They'll use your child's Social Security number with a different name and a different birth date," Holland said. "So if you pull a credit report, the credit report is looking for a specific name and the birthday that goes with it. And so you won't find it. You'll get "file not found," and you'll feel safe."

"The problem is large and growing," said David Vladeck, the FTC's director of the Bureau of Consumer Protection. "Part of the problem is it's undetected and undetectable."

Authorities advise parents to:

• Make sure you have antivirus software installed on your home computer.
• Tell your children never to give out their Social Security number without your permission.
• Check your children's credit periodically, even when they are under age.

"Parents need to understand that there are measures they can take to safeguard their children's identity," said Vladeck. "Parents should think about protecting their children's identity, and the Social Security number is absolutely the foundation there."

Enjoli Francis and Jack Date contributed to this report.