How do you say 18 years old in mandarin?

There are a couple of ways to say 18 years old in Mandarin, depending on the context:

Formal:

* 十八岁 (shíbā suì) - This is the most common and literal translation.

Informal:

* 十八 (shíbā) - This is a more informal way to say 18, especially when referring to a person's age.

In a sentence:

* 他今年十八岁。 (Tā jīnnián shíbā suì.) - He is 18 years old this year.

* 她今年十八了。(Tā jīnnián shíbā le.) - She turned 18 this year. (More informal)

Remember that the tone of the numbers is important in Mandarin. "十八" should be pronounced with the tones shí-bā (2nd tone, 1st tone).