Adolescence (5)
(Other definitions for teens that I've seen before include "Adolescence" , "children" , "Years 13-19" , "13-19 age group" , "seven-year period" .)
'adolescence' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for youth that I've seen before include "Adolescence and early manhood" , "period before getting old" , "Adolescent" , "Young man" , "Youngsters" .)