Volume Bisky's age is revealed: April 2002
Volume Tsunade's age is revealed: August 2003.
The trend of Naruto Part 1 taking things from HunterxHunter never ends. A big part of why Shippuden sucks so much is because Kishimoto ran out of HunterxHunter chapters to copy thanks to HiatusxHiatus.
I'm not worried, actually. Just in general that's how I feel about characters in anime that look younger than they are. Usually it's done for gross purposes, but when it's not I have no problems with it.