Two words: Individual basis, when you stop acting like a teen, you are no longer a teen for me. You could be 30 years old and still be considered a teen in my eyes, likewise, you could be 15 years old and not be considered a teen to me for many reason, such as being raised in a culture where adolescence is near nonexistent. But if I were to generally go by my American culture and how I see things, people are usually no longer teens in between 18 and 20. They may still have teen tendencies, but many try to become more adult during this time, at least in my experience.