One Piece isn't meant for children any more than Naruto is.

Also, Autophage, your point becomes quite moot because Blackbeard (or Blackmoustasche...) doesn't even have a moustasche.
He's a bit unshaven yes, but it's not a moustasche like the one Whitebeard has.

I am also quite sure that "beard" was Oda's idea of translating hige because *color*beard names were typical of pirates.