Youth is usually defined as being under 25 years old in some places but consider that youth ends at age 21 when you reach full adulthood.
Still you can feel youthful at any age even though technically you're no longer an actual youth.
Early Adolescence is around 14 and under and middle Adolescence is 15 to 17 and late Adolescence is 18 to 24 years of age.
I no longer considered myself a youth though when I turned 18 but my parents still thought of me as a youth.