according to some, “teenager” is a flawed modern concept. By halocha, a child is not responsible, his parents are. Then, at bar/bas mitzvah, he is. Teenager is a creature with partial responsibility and leads to finding excuses to evade responsibilities. There were no teenagers before modernity and the world went round and round. I am not sure I 100% subscribe to that, but there is something here.
according to others (R Orlowek), recently teeangers start at about 8 due to access to information, so treat them accordingly.