Child = A person younger than 18 years, unless majority (ie, the legal threshold of adulthood) is attained at a younger age in a particular country. (The Convention on the Rights of the Child, 1989)
Adolescence = The period between ages 10 and 19 years. It is derived from the Latin adolescere—the present participle adolescens means growing up, whereas the past participle adultus means grown up. (WHO)
Youth = People aged between 15 years and 24 years (UN, 1985)
Teenager = People aged 13–19 years.
Young people = People aged 10–24 years (ไม่ค่อยนิยมนิยามคำนี้) มักจะกล่าวถึงในลักษณะเช่นนี้ในรายงาน โดยแบ่งออกเป็น 3 กลุ่ม ได้แก่ (1) 10–14 years (early adolescence) (2) 15–19 years (late adolescence) และ (3) 20–24 years (young adulthood