The youths were X-rayed to try to determine their real age
Bone tests on teenage athletes in south China have shown that thousands had faked their age, often in order to keep competing in junior events.
Tests on nearly 13,000 athletes found that more than 3,000 were older than their registered age, according to the Sports Bureau of Guangdong Province.
At least one athlete was seven years older than their stated age, but most were said to differ by a year or two.
The news comes as Guangdong prepares to host the 2010 Asian Games.
The investigation is the latest in a number of initiatives by the Chinese authorities to crack down on the practice of age-faking, which many experts believe is rampant.
The expensive bone age analysis tests were carried out on teenage athletes registered with sports academies in Guangdong.
The province's governing body found that about a fifth of those tested had lied about how old they were.
"We must ensure that those athletes faking their ages cannot find any way to take advantage [in competition]," officials were quoted by local media as saying.
"Based on the bone X-ray examinations, we will review all the results of youth sports competition in 2008."
Funding of sport at provincial level is dependent on success.
The BBC's sport news reporter, Alex Capstick, says local officials are under huge pressure to win, which makes them more likely to bend the rules.
It is no surprise some athletes and their families, many of whom see sport as a way out of hardship, have joined in the lie as the system only rewards the very best, our correspondent says.
Chinese athletes have faced repeated claims of age-faking in recent years.
At last year's Olympics in Beijing, some of China's gold-winning gymnasts were alleged to be below the minimum age of 16.
However, after an inquiry, the sport's governing body cleared them of any wrongdoing.
The Chinese Basketball Association recently announced that last year 26 players in the top league had registered an incorrect age. This would have allowed them to represent junior teams when they were in fact too old.
There have been similar problems in football.
At the weekend, it emerged that a badminton player who had won a provincial title as a 14-year-old had now admitted to being 17 at the time of the contest.