Question

At approximately what age can a child first follow the principles of grammar most of the time?

Answer 1

At three years of age, a child can mostly do this.

Answer 2

The person above me is correct - at approximately 3 years of age can a child first follow the principles of grammar most of the time.
Granted, often he will use these principles wrong, for example, the child may not yet be aware of irregular plural forms, and might say childs instead of children, or irregular past tense, so they might say seed instead of saw.