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We all know we change after some years. When we become adults, we do not think the same way as we did when we were children. We change our habits, hobbies and concerns. We become mature. We start to act in a different way.
When you are teaching English as a second language, you constantly deal with different age level classes. You can deal with kids, teens or adults.
During your experience as a teacher, you are going to find out that each age level has its own characteristics.