Okay, so the edit reasons still doesn't appear to be working properly so I'm reiterating this point.

• One Piece kind of follows this, maybe. No one in the series has aged since the story began ten years ago; word of god says that Luffy celebrates his birthday every year, but it's always his seventeenth. Of course, Oda is known for joking around in his letter columns so this is (probably) a gag.
  • This might be moot anyway, since depending on how you interpret events, as little as two months may have passed in in-universe time since the series began, so it's possible no one has had a birthday yet, anyway. On the other hand, it's also possible that years have passed. It's impossible to tell for sure because we're never told how long the crew spends sailing from place to place.
    • The Roger Pirates took three years to sail around the Grand Line, and the Straw Hats are around halfway though, although considering it can take up to a year for the Log Pose to reset, they may have taken longer routes than the Straw Hats did.
  • Luffy is so stupid he may very well celebrate his seventeenth birthday every year.
    • Considering the series, you'd expect Luffy saying that he celebrates his 17th birthday every year is what keeps time standing still.
  • Word Of God has also said that the current storyline will be "Luffy's last adventure as a 17 year old" though it's currently up in the air whether he means this literally or figuratively.
    • And now he's 19.

According to comments I've heard, a decade really hasn't passed, nor presumably has evidence of that amount of time. Also, who of this manga's cast are children?

I think this (http://www.youtube.com/watch?v=tVi3vflC54g&feature=player_profilepage) lampshades the "Not Allowed To Grow Up" trope at the very end.