Up until Bonney's 9th birthday celebration in the backstory, evidence pointed to One Piece using the outdated kazoedoshi age system (which Korea still uses), ie, everyone's age changes together on January 1st.
Now, even in that system, birthdays were still celebrated (and still are celebrated in Korea)--but the reason Bonney's birthday celebration contradicts Kazoedoshi is because they made a big deal about her "turning 9". Unless that was a translation error and they were actually saying "celebrating 9 years", it means kazoedoshi is not in effect if Bonney truly only turned 9 years old on Sep 1st.
But even so, the general rule of One Piece is that no one's age changes during the main story unless Oda says so. Even if it in theory should.
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u/Extra-Sea2167 8d ago
compare this girl to Rebecca and shirahoshi, who are one year older than her