The state or quality of being adolescent; the period of adolescence.
From 'adolescent' plus the suffix '-cy' meaning 'state or condition.' This noun form emerged to describe adolescence as a particular life stage with its own characteristics.
'Adolescency' is basically a fancier, more formal way to say 'adolescence'—some older texts use it, but it's mostly disappeared because the simpler form works just fine.
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