Beating a 12-year-old rival was once child's play for an elite swimmer, but it has become an unlikely badge of honour when China's Yu Zidi is in the race.

Only the very best in women's swimming were able to beat Yu at the world championships where the prodigy became the youngest medallist in the global meet's 52-year history.

Yet even as the swimming world marvelled at Yu's talent, her age raised questions about the ethics of someone so young competing at the highest level.

In a sport where the mental and physical toll can prove overwhelming for adults, Yu's success at the world championships was viewed dimly by some child protection advocates.

"Twelve-years-old is incredibly young in swimming," author Linda Flanagan, a vocal opponent of children competing in elite sport, told Reuters.

"A 12-year-old does not train like an 18 or 20-year-old because their bodies aren't equipped to handle the work involved.

"And also emotionally, it’s not in their best interest.

"It flies in the face of what psychologists say about what’s healthy for kids, which is not to put all your eggs in one basket, to spread your interests so you don’t become this narrow person."

Adolescents are hardly rare in swimming, particularly in women's competition where athletes tend to reach peak performance earlier than men.

Dane Inge Sorensen was 12 when she won the 200 metres breaststroke bronze at the 1936 Berlin Olympics. She remains the youngest Games medallist in individual events.

Sharron Davies swam for Britain at 11 and made her Olympic debut at the 1976 Montreal Games at 13.

Far from being concerned about Yu's wellbeing, Davies said the Chinese wunderkind, who won a relay bronze and came fourth in three individual events on her world championships debut, would have found it a thrill.

"It is a total misconception that that young girl would have been extremely nervous. It would have been the opposite," Davies told Reuters in an interview.