We are told that Waverly's family receives the chess set when they went to the annual Christmas party held at the First Chinese Baptist Church. There was a Santa there with a sack of presents, and it is when Waverly goes to Santa to receive her present that we find out how old she was. Note how the narration gives yet one more example of the complex, hybrid world that Waverly inhabits, for she is not sure how to answer the question:
When my turn came up, the Santa man asked me how old I was. I thought it was a trick question; I was seven according to the American formula and eight by the Chinese calendar. I said I was born on March 17, 1951. That seemed to satisfy him.
This clearly shows how confusing the position of someone like Waverly is when you belong to two different worlds but aren't sure of how to position yourself.
However, even though it is Waverly's older brother Vincent who gets the chess set, it is when Waverly was seven (according to the American formula) that she started to learn how to play chess.