text: If you look at developmental theorists like Piaget, Freud, Eriksen then "youth" is a separate stage on our path to adulthood and old age.  You could even say there are individual differences in particular behaviour such as criminal behaviour etc and you could say that there might be individual differences in some aspects of personality such as locus of control.  So, in that respect, yes, it could be a clearly definable stage although you would have to be clear on what the definition of "youth" is.