A teenager, or teen, is a person who falls within the ages of 13 to 19 years old. The word "teenager" is often associated with adolescence. However the World Health Organization considers anyone between the ages of 10 and 21 to be an adolescent, and most neurologists consider the brain still developing into the early third decade (early 20s). After the 20th birthday, one is no longer a teenager. However, neural plasticity continues, as the brain and particularly the prefrontal cortex (PFC) develop further.