Don't forget the cultural aspects of determining age...

When asked for age, in the US, most say "I am nn years old", filling in the number from the last birthday (taking into account the variations described by Wen above...).

In England (and maybe the rest of the UK?) "I will be nn years old", filling in the number for the upcoming birthday.

The Chinese have their own way of calculating age, and I'm afraid I don't understand it well enough to try to explain it. (Any member of Chinese extraction, please do explain it to us!)

How about the rest of the international membership--how does your culture calculate and respond to the answer to the question: "How old are you?"