Have you ever wondered why it is so easy for young children to learn languages, and why as adults it can feel incredibly challenging? This is because children acquire language, whereas adults learn the language.
Children between the ages of 1-12 still have a very plastic brain. They are able to simply absorb information, and make sense of it on their own. They don't need somebody to sit down with them and explain the grammar rules to them. This is why your parents never needed to explain the grammar of your native language to you - you were able to subconsciously interpret the language and begin using it yourself. It is a really amazing ability, and one that I wish carried with us into our adult years!
Adults, on the other hand, must learn language. After age 12, your brain becomes less absorbent. Your neural pathways become more solidified, and it is much harder to process new information, such as a new language. The way you learn changes: you must be taught the rules of a new language, which your brain then organizes into the schemas it already has. You view your second language through the lens of your first, because that is the language that you know. Your brain is no longer able to absorb a second language and place it into a category of its own, but instead must organize the new information by viewing it through the grammar structures of the first.
When I learned Spanish, I was older than 12. English is my first language, and so listening to people speak in Spanish felt incredibly foreign. Now, had I begun to learn Spanish at 4 or 5 years old, I would have gradually been able to pick it up, without needing it explained to me. Instead, I took Spanish language classes throughout high school and university, and am now living in Colombia in order to improve my fluency.
This is why the way we teach adults language and the way that we teach children language varies greatly. Children are able to absorb information more easily, but also have a much shorter attention span. So while you can provide them with more information, you need to vary the way in which you show them language so that they don't lose interest. Adults, on the other hand, are able to focus for much longer on one topic, but you simply cannot throw the same level of information at them. It is important to focus on lots of repetition with both children and adults, but with adults it becomes crucial due to their brain's ability to take in new information.