International gymnastics officials are examining whether Yang Yun and Dong Fangxiao, in particular, were old enough to compete.
"If we had a look at all the articles that came before, during and after the games, there were always rumors about the ages of China's athletes in Sydney," Andre Gueisbuhler, secretary general of the International Gymnastics Federation, told The Associated Press on Wednesday.
"We did not have another choice," he said. "If we want to remain credible, then we have to look into things."
No other Chinese teams are being investigated, Gueisbuhler said. "At this moment in time, we just have concerns about 2000 and 2008," he said.
The investigation is ongoing, a month after the Beijing Games ended, and there is no timetable for when it will be finished. "It's a work in progress," said Emmanuelle Moreau, spokeswoman for the International Olympic Committee. "Until the work has been completed, there is nothing we can say."
Zhang Hongliang, a media officer for the Chinese Gymnastics Center, which oversees the Chinese national team, said he didn't know anything about an investigation into the Sydney athletes. The center would "promptly provide information" to the FIG, Zhang said.
Yang's age has long been an issue for debate.
In a June 2007 interview that aired on state broadcaster China Central Television, Yang said she was 14 at the Sydney Games. Gymnastics rules require athletes to be 16 during an Olympic year in order to compete.
"At the time I was only 14," she said in the interview, done in Chinese. "I thought that if I failed this time, I'll do it again next time. There's still hope."
That interview, which has been widely reported, contradicts her official birthdate, which is listed as Dec. 2, 1984, and made her eligible for Sydney. Yang's agent, Zhu Shuangshuang, didn't immediately respond to messages seeking comment.
Dong's birthdate is listed as Jan. 20, 1983, making her 17 at the time of the Sydney Games. Her blog, however, includes a reference to being born in 1985.
Yang is now engaged to Olympic all-around champion Yang Wei. Dong was a national technical official at the Beijing Olympics, serving as the secretary on vault. She was not part of any judging panel.
Kui Yuanyuan, Ling Jie, Liu Xuan and Huang Mandan were the other members of China's 2000 squad.
Age falsification has been a problem in gymnastics since the 1980s, after the minimum age was raised from 14 to 15 in an effort to protect young athletes, whose bodies are still developing, from serious injuries. Younger gymnasts are also thought to have an advantage because they are more flexible and are likely to have an easier time doing the tough skills the sport requires. They also aren't as likely to have a fear of failure.
The minimum age was raised to its current 16 in 1997.
There were questions about the ages of China's Beijing squad months before the games, with media reports and online records suggesting several of the gymnasts on the six-woman squad might be as young as 14.
In August, The Associated Press found registration lists previously posted on the Web site of the General Administration of Sport of China that showed He Kexin and Yang Yilin were too young to compete. A Nov. 3 story by the Chinese government's news agency, Xinhua, suggested He was only 14.
But Chinese officials insisted - repeatedly and heatedly - that all of its gymnasts were old enough, and they had not cheated their way to their first Olympic team gold. The FIG and IOC hoped the matter had been put to rest before the games, when the IOC said it had checked all of the girls' passports and found them to be valid.
The controversy never went away, though, and the IOC announced three days before the games ended that it had asked the FIG to investigate one more time.
China turned over birth certificates, passports, ID cards and family residence permits for He, Yang, Jiang Yuyuan, Deng Linlin and Li Shanshan.
"The international federation has required the delivery of birth certificates and all the documents like family books, entries in schools and things like that," IOC president Jacques Rogge said on the final day of the games. "They have received the documents, and at first sight it seems to be OK."
If evidence of cheating is found, it could affect as many as four of the six medals the Chinese women won in Beijing. In addition to the team gold, He won gold on uneven bars and Yang got bronze medals on bars and in the all-around.
"We are waiting to hear the outcome of the IOC investigation just like everyone else," said Steve Penny, president of USA Gymnastics.
AP Sports Writer Stephen Wilson in London and Associated Press writer Anita Chang in Beijing contributed to this report.