Tags : :
The Times, 11 August 1965
[article discussing 100m backstroke world record by 12 year South African schoolgirl Karen Muir]
"It is very rare to find a sports international at 12, although Joy Foster did play table tennis for Jamaica at the age of eight in 1958..."
[NFM: she also won the Jamaican championships in women's singles, women's doubles and mixed doubles the same year]
So Joy Foster seems to be the youngest person ever to represent her/his country in international sporting competition (at senior level - not counting youth competitions obviously).
I found only one potential exception : in the 1900 Olympic coxed pairs rowing, the Dutch pair replaced their regular coxswain, who was considered too heavy, with a French schoolboy. They won the gold medal, but the boy's name has not been recorded (it is also not clear whether he was given a medal). Some sources say that he was 7 years old. In any case he was not quite representing his country I suppose.
I thought perhaps gymnastics would have a younger representative, but apparently in female gymnastics the youngest was Pasakevi 'Voula' Kouna of Greece, aged 9, and in mens Dimitris Loundras (also Greece) aged 10. The latter won a bronze medal in the 1896 Olympics.