Experience does not necessarily mean a person will be a good teacher. Yesterday I heard about a 17 year old who has summited the highest peaks on each continent (including Everest, naturally). Experienced? Certainly appears so. Is he capable of teaching? I don't know. An instructor not only needs experience, but needs the ability to meaningfully translate that experience into material capable of being understood, accepted, and implemented by another. Most--but definitely not all--18-year olds I know lack that ability.