The word sai (才/歳) is used to count age in Japanese and can be written with either kanji. Some common ways to ask how old one is in Japanese are,
What age are you = nan sai desu ka (何歳ですか)
How old are you (polite) = oikutsu desu ka (おいくつですか)
How old are you Jim? (casual) = jimu-san, ikutsu?
Like other counters in Japanese the word follows its own pattern. Please memorize how to say each number because it doesn't follow the same pattern as regular Japanese numbers. The pattern you see continues past age 12, except for 20.
Age twenty is pronounced hatachi. In Japan when you turn 20 you are considered an adult. Even though it has its own special pronunciation you can also say nijuusai.
Seijin no hi (成人の日) is a Japanese holiday that is healed on the second Monday in January each year. This day is to congratulate those who have turned 20 over the past year.
Special ceremonies (seijin shiki) are generally held in the morning at local city offices. Many women celebrate this day by wearing special style of kimono, gorgeous hair and make-up. Men sometimes wear a traditional mens kimono, but now a days men often wear a western style suit.