If I'm not wrong (and based on a Sailor Moon episode) there are come traditions in Japan about turning 20 and becoming an adult to the eyes of society or something like that
edit: Found this note on Namie's Sweet 19 Blues' review at Allmusic
Quote:
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Amuro's debut album came almost presold; four of its songs had already been number one hits in Japan's Oricon charts, all before the teen idol had hit her 19th birthday. (The melancholic passing of another sweet year of youth -- a particularly Japanese obsession -- was the subject of the album title and accompanying song.)
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