Years of youth (5)
I believe the answer is:
teens
'years of youth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for teens that I've seen before include "13-19 age group" , "The years 13 to 19" , "Years 13-19" , "seven-year period" , "Between 12 and 20" .)