Im 21. Time for a little graph.
My interpretation:
Many readers form the big bulk that was around 16,17 five years ago are still around and pull the average age up. Also there's a new group of readers around 15 years old today, many of them probably got into One Piece via non traditional media. Sadly the methuselahs from 2005 seem to be gone. The age group of 30+ is still almost non existant. Maybe fans that old don't waste their time surfing message boards anymore…