Asking someone their age

• How old are you?

  Nansai desu ka?
• i am (years) years old.

  (years)sai desu.
• 3 years old

  san sai
• 15 years old

  juu go sai
• 100 years old

  hyaku sai
• Mr. Smith: How old are you?

  Mr. Hayashi: I am 25 years old.

  Mr. Smith: Nansai desu ka?

  Mr. Hayashi: Ni juu go sai desu.