3 years
In the third year simple sentences become more complex as children show early signs of understanding adult grammar. They begin to use tenses other than the present tense i.e. "I kicked it". The sentences they produce become more complex. By age 3, most children learn new words quickly and can follow two-part instructions (such as "wash your face and comb your hair").
They start to use plurals and form short complete sentences. And most of the time their speech can be understood by others outside of their family. In particular "why?" and "what?" and "who?" become popular questions. An important feature of questions is that they enable children to learn new things.