At what age have most children learned the basic grammar of their language?

Most children have mastered the fundamentals of the systematic grammar of their native language by about the age of four.1

While language development is a progressive process that begins in infancy, by the time a child reaches age four, they are typically able to converse in adultlike sentences. By age five and six, children further demonstrate metalinguistic awareness, which includes the ability to distinguish between grammatically correct and incorrect sentences and the capacity to make spontaneous corrections to their own speech.2