Sentence:  The 18-year-old Bieber can't quite pull off the “adult” thing just yet: His voice may have <WSD>dropped</WSD> a bit since the days of “Baby,” but it still mostly registers as “angelic,” and veers toward a pubescent whine at times. 
Meaning of the word marked with <WSD> tags:  To lower in timbre, often relating to puberty.