I've been wondering this for a while, because no voice actor (Especially Luffy's) has shown any particular change in voice nor have the characters made any massive physical changes, save for costume changes. As cool as costume changes are, I'm a bit concerned about the ages of the characters.
I mean, remember that… Oh, whoops. Manga spoilers.
[hide]Coby has changed alot. Like… alot. That kind of puberty can't be topped with at least 5 to 7 years.[/hide]
No one can blow time itself out of proportion like Oda can.