A person becomes a teenager when they become 13 years old. It ends when they become 20 years old. Teenagers who are between 14 and 17 years old are considered both children (in English-speaking countries) and teenagers in most countries. Teenagers who are 18 and 19 years old may be considered both teenagers and adults, although they're considered biologically adults before that, around the 16 years (counting both sexes together).