I actually love the chinese zodiac and if you do not know someone's age but know which zodiac they belong to, you can work out their age accordingly. The zodiac runs in cycles of 12 years, so if someone is born in the year of the rabbit, then their age is in multiples of 12 this year e.g. 12, 24, 36, 48.
The story of the race behind the order of the animals is also interesting and the animals are listed in the following order:
The ferocious tiger is exiting to make way for the cute rabbit in a few days time. But the next year of the dragon would likely see a surge in the baby births because it is thought to be auspicious to have a dragon child. A link is shown below on the chinese zodiac: