To be honest, One Piece is clearly aimed at an older audience, and 4Kids tried to...well, make it 4Kids.
No, One Piece was in Shonen Jump which is aimed at young boys who crave bloody bouts and the like. Japan's censorship is just a lot more... lax compared to America.
Japanese Ash has more emotion and says lines that would come out of a 10-year old (strange plot structure and character development aside), but dubbed Ash is almost a robot or a very genki kid with lines that should belong to an adult (which you can clearly compare when he was stuck with Lysandre and Alain on a tower for 3 episodes).
With 4kids, they actually were able to be relatively faithful to the original while adapting for English viewers. Though in some cases, they'd go a tad to far, I'd say Ash back then could sound like a kid more.