NHK's Modern Close-Up program had a One Piece special called, Secrets of the Mega-Hit One Piece which list the statistics of its buyers coming from Kinokuniya Bookstores which reports that most of the buyers are adults!
According to the translation of the chart seen at the program (above), 43% of the buyers are from ages 19-29, 32% are from ages 30-49%, 12% are from ages 1-18 and surprisingly, 13% of the readers are from ages 50 and above! The program concluded stating the reason on the amount of adult reason is that the relationship of mutual trust among the characters attracted the said demographic, who have been exhausted with the economic recession.
Source: Yaraon!, Comic Natalie via ANN, MyAnimeList