When it comes to one's age, in Chinese astrology - and even in general conversation with a Chinese individual - it is usually given as at least a year more than in western tradition. For instance, a child born today will, in Chinese terms, turn two at the start of the next Lunar New Year, which falls on January 26. By western calculations he or she would only be 21/2 months. Note that others take into account the winter solstice as the start of a new year, which means that everyone becomes a year older on December 21.
This discrepancy can be attributed to the belief that when a child is born, he is already nine or so months old. There are actually Chinese terms that take into account this difference: one's age in western terms is known as the shi or 'real' age while the Chinese age is known as the xu or 'empty' age.
Bear in mind that you'll have to add a year to your age when consulting Chinese astrologers or reading astrology manuals.