A couple years ago, a good friend commented on the brain-dead stage her daughters had gone through when they were 12 to 13 years old. I think it has been the most useful information I've ever had for parenting teenagers; they all seem to go through this brain-dead stage, although for boys it tends to be around 15 years old. I think it has to do with the full flush of adolescence, as they adjust to their new body and their brains try to catch up.
How to tell if a teenager is in the brain-dead phase: They forget that you told them about grandparents coming for dinner. They definitely forget chores that you have told them about three times (or more). They leave coats, cell phones, or clothes at friends' houses or in cars. They have trouble focusing on schoolwork. They get school assignments done but forget to turn them in. It can be infuriating, but they really aren't doing it on purpose - they just don't have enough brain cells available to cope with their physical and emotional development AND with the outside world. Knowing that it is just a stage is tremendously reassuring; it's nice to know that my formerly-competent child is just absent for a year or so, not gone forever.