By age ten, your child can comprehend the abstract grammatical concepts of noun, verb, participle, and gerund. English grammar, or any language grammar, can be readily learned.
Some begin the formal study of English grammar in the first grade (age six). We would recommend beginning at age ten (grade level five). The abstract concepts of formal mathematics and formal grammar are best left until the child is developmentally most ready to handle them. Before age ten, memorization, oral narration, reading aloud, and copywork will build a solid foundation for the study of formal grammar later.
There is a difference between learning a language and learning grammar. Children begin to learn their native language from their earliest age, and they can develop skill in speaking several languages while they are still in their youngest years. Indeed, this is the optimal time for developing spoken proficiency in a language. Formal grammar, how- ever, is the study of the structure of a language, and such study should be delayed until the brain is developed to handle it.
One part of the brain handles the language, and another part of the brain handles the grammatical analysis of the language. The part which handles the language is well developed by age four or so. The child learns inductively that the subject comes before the verb and the direct object comes after the verb, even though he has no way of conceiving what a subject, a verb, and a direct object are. He learns vocabulary and style without any way of conceiving what a noun, a verb, a preposition, or what iambic pentameter is. He just enjoys language. The part of the brain which handles the formal grammar is developed by age ten or so. If you force formal grammar too early, then you will put the information in odd places of the brain, and it is more difficult for the brain to assimilate and to make use of the information.
Three years (ages ten through twelve) for studying English grammar are usually sufficient. Your child will be studying Latin or Greek grammar by age thirteen, which renders the study of English grammar largely superfluous.