Re: How can it be definitively proven that these girls even know how old they are?
If you live on an island you may not know your true age.
In China??
Cmon.
One other thing of which I am unsure--my best friend while growing up had been born in Saigon, came to the US in 1975. We were both born in the same year, but she always quoted her age as a year older. She explained to me that it was traditional, in Vietnam, to give your age as the year you are beginning to live, not the year you have just completed. For all I know this is common throughout Asia. Might be just another way to confuse an already confused young girl about her age.
I don't know about you, but I spend an awful lot of time with 3-4 year olds. They'll believe practically anything.