As with telling the time, the verb 是 shì to be is not used when talking about years of age:
你弟弟 几岁 ?
H ow old is your younger brother ? (lit. your younger brother how old)
我 五 岁 的 时候 每 天 都 吃 中 国 饭
wǒ wŭ suì de shíhou měitiān dōu chī Zhōngguó fàn
When I was five years old, I ate Chinese food every day.
Translate the following sentences into Chinese (20, age)
I'm 20 this year. How old are you?
I'm 21. When is your birthday?
30 January, and you?
It's my birthday too.
How old is your Chinese teacher?
I don't know you should ask him.