The U.S. women, who had been fourth, move up to the bronze.
The IOC said Dong was also stripped of her sixth-place result in the individual floor exercises and seventh place in the vault.
Calls to the Chinese Gymnastics Association and the media officers for the Chinese gymnastics team went unanswered late Wednesday. Dong now lives in New Zealand with her husband.
The IOC ordered China's national Olympic committee to return the team medals "as soon as possible" so they can be reallocated to the U.S. team.
The IOC also told the Chinese to "ensure, by all means, that the athletes and officials of its delegation comply with all rules and regulations (of the international federation) particularly with regard to age limits."
Questions about Dong's eligibility arose during the FIG's investigation into the ages of China's team that won the gold medal at the 2008 Beijing Games. Media reports and Internet records suggested some of the girls on that team could have been as young as 14.
The FIG cleared the Beijing Games gymnasts in October 2008 after Chinese officials provided original passports, ID cards and family registers showing all of the gymnasts were old enough to compete. But the FIG said it wasn't satisfied with "the explanations and evidence provided to date" for Dong and a second gymnast, Yang Yun.
Dong's accreditation information for the Beijing Olympics, where she worked as a national technical official, listed her birthday as Jan. 23, 1986. That would have made her 14 in Sydney -- too young to compete. Her birth date in the FIG database is listed as Jan. 20, 1983.
Dong's blog also said she was born in the Year of the Ox in the Chinese zodiac, which dates from Feb. 20, 1985, to Feb. 8, 1986.
FIG investigators didn't find sufficient evidence to prove Yang, who also won a bronze medal on uneven bars in 2000, was underage. She received a warning from the FIG.
The bronze medal salvages what had been a disappointing Olympics for the U.S. women. The squad -- Amy Chow, Jamie Dantzscher, Dominique Dawes, Kristin Maloney, Elise Ray and Tasha Schwikert -- left Sydney empty-handed, the only time since 1976 the American women had failed to win a single Olympic medal. The U.S. boycotted the 1980 Moscow Games.