My club had a number of teens in Rich Lewis's club during the 90's. Then, well, they weren't teens anymore. I wouldn't say they should be taught as adults. I'll talk about two of them. There were a pair of brothers experienced with 10 years of TKD before starting this club. They could't take the beating as much and would bawl pretty much every class. Their Dad thought this was a good source of discipline because Rich ran his class military style, really, as he says he did when he was in Special Forces during the 60's overseas. One brother didn't last long even though his build was bigger. Overall, their memories weren't as strong, so they memorized fewer moves and needed more reminders. However, I'd also say that this was probably specific to these brothers. There were a pair of teen sisters that trained for about five years, and they loved the art. However, push them too hard or speak to them too sternly and the tears would flow. So, I wouldn't say treat them as adults, especially female teens. There's a reason you have to be at least 15 to have an adult grade.