First, never answer this question, it’s a trap!
Then, is there a real answer to this question? I mean, can you really choose an age from which you’re not young anymore? Let’s say that this age is 25 years old. It means that when you turn 26 you enter in the circle of the old people? I don’t think we are living in a world like this.
Let me explain.
To start, we could maybe take a look at what the organisations like the United Nations say about the youth’s age and the definition. They defined youth as a period of transition from the dependence of childhood to adulthood’s indépendance and awareness of our interdependence as numbers of community and for statistical consistency across regions, it defines « youth », as those persons between the ages of 15 and 24 years.
After this definition, we can see that according to their standards, youth lasts almost ten years but youth is also a notion hard to contain in only one definition. This is why several psychologists wrote about this important topic, trying to define it from different points of vue.
Erik Erikson, one of the most popular psychologists is one of them. In his 'stages of development' he describes the stage of identity vs. role confusion which is said to occur from age 12 to age 18, as a stage in which the individual re-examines his or her identity and tries to find out exactly who he or she is.
At the same time, Sigmund Freud, in his theory of stages of psychosexual development stated that the fourth and the last stage of development was the genital stage. This stage is set to begin with adolescence (regarded to begin at age 12) and continue throughout the adult years. No specific age at which the period of youth beings and ends were given by him.
Carl Jung, another giant in the field of psychology, finally stated that the period of youth is a period in which the demands of life harshly put an end to the dreams of childhood and that if an individual is well prepared then the transition to a career can take place smoothly but if the youth is stuck with illusions which are the opposite of reality, then problems arise. To be more specific, Jung defined youth as a period which begins at age 15 and ends at age 40.
As you can notice, depending on the psychologist or, in general, the person, youth doesn’t really fit in one fixed period but it’s more something subjective where everyone has a different opinion and way to think about it. Youth is a large notion so of course you can read and listen to what the professional thinks and says about it but you can also think by yourself and define your own perception of youth.