AGE

To express age in Japanese, you use the number of years + the counter SAI

How old are you?              -         nansai desuka?

(other alternative way of asking the same question - oikutsu desuka? However, the answers have no alternative style)

How old is she?                 -         kanojyo wa nansai desuka?

The suffix SAI is never dropped.

I am thirty-one (years old)  -         san-jyuu-issai desu.

The only exception is 20 which is HATACHI 

(remember, you cannot use SAI for the age of things, only people)

This car is two years old     -         kono kuruma o ni-nen mae ni kaimashita.

This building is one hundred years old   -      
                                                       kono tatemono wa hyaku-nen mae ni tateraremashita.
                                                      (This building was built 100 years ago)

A person in their fifties        -         go-jyuu-dai no hito

A person in their seventies  -         nana-jyuu-dai no hito

People over eighteen          -         jyuu-hassai ijyoo no hito