The Chinese term for 13 is "ten three" (the word for "ten", followed by the word for "three"). Furthermore, every digit is one syllable. It really does make learning easier for children. The ease of learning is even more apparent when learning multiplication, since there is a certain rhythm when you recite the multiplication table.
Gladwell actually does address the concept of cumulative advantage, as applied to sports. Kids with January birthdays are more likely to do well in children's sports leagues, and the slight advantage they have at every stage in their sports training leads to a preponderance of professional athletes with January birthdays. Likewise, Chinese children learn to count earlier, start learning multiplication around first grade, and can move on to more advanced topics. This is not to say that all Chinese people are good at math, though. There still exist analytical and artistic students, and generally Chinese school systems allow students to specialize in math/science or the arts after elementary school.