A teenager, or teen, is someone who is between 13 and 19 years old. The ages vary depending on each country. They are called teenagers because their age number ends with "teen". The word "teenager" is often associated with adolescence. Most neurologists consider the brain still developing into the persons early, or mid-20s. A person begins their teenage life when they become 13 years old, and ends when they become 20 years old. Teenagers who are 18 and 19 years old are, in most nations, both teenagers and adults. 