Ch. 2: Nan-sai desu ka? (How old are you?)
To say your age, find out what the number is first and then add -sai to the end.
A person who is 10 years old would say: Watashi wa, juusai desu. 私は、十歳です。（わたしは、じゅうさいです。）
A person who is 15 years old would say: Watashi wa, juu-gosai desu. 私は、十五歳です。
(わたしは、じゅうごさいです。）
Reaching the age of 20 qualifies you as an official adult in Japanese culture, so when someone is 20, they say: hatachi desu. 二十歳です（はたちです）
issai - 1 year old
nisai - 2 years old
sansai - 3 years old
yonsai - 4 years old
gosai - 5 years old
rokusai - 6 years old
nanasai - 7 years old
hassai - 8 years old
kyuusai - 9 years old
juusai - 10 years old
***juuissai - 11 years old
***juuhassai - 18 years old
***Note that when you run into a situation where -chi is followed by -sai, they gloss together.
Example:
nijuu-issai - 21 years old
nijuu-hassai - 28 years old