Children need direction in the learning process. For most children, a lack of structure and directed study guidance results in chaos, a lack of focus and a lack of learning the required material. Adults work in a different method. Adults are used to being self-directed and need to be encouraged to guide themselves when studying. Teachers take on the role of facilitator more than instructor when dealing with adult students. Ultimately, an adult has to be responsible for his own learning.
Children find it easy to learn things by rote. They don't have a lifetime of life experience to frame their education, so often the easiest way to teach a child is to simply present them with information in a number of different ways until they memorize it. This is more difficult with adults. Adults tend to require a purpose or direction in their studies in order to process the new information within their existing frame of reference.
Teachers need to understand what adult students understand about a given topic in order to teach effectively to a class full of adults. When teaching young people, it is easy to assume or deduce what students know. For instance, a high school chemistry teacher teaching freshman chemistry knows that all the students know only what they have learned in general junior high science classes. On the other hand, a teacher teaching an adult marketing class could have a person with no marketing experience in class, and one seeking a degree to augment 20 years of work experience. Teachers are more effective once they know how much an adult student knows about a given subject.
Children in class are not concerned with time except to the extent that they want class to be over so they can go out and play. However, the real value of time is not something that becomes apparent until adulthood. Adults value their time greatly, and have to weigh time spent learning against other responsibilities. Direct, applicable knowledge is of more interest to adults as they don't typically want to waste time on broad topics or generalities.