Okay, I'm going to take a shot at this.
The third databook lists birthdays AND current ages. So let's see if we can do the actual break down using both the modern Japanese school year and the calendar year.
Academy Class by Japanese School Year
Class Year 1 :: April 1 -- March 31
Neji 17 07/03
Sai 17 11/25
Lee 17 11/27
----------------------------------
Tenten 17 03/09
Class Year 2 :: April 1 -- March 31
Choji 16 05/01
Kiba 16 07/07
Sasuke 16 07/23
Shikamaru 16 09/22
Ino 16 09/23
Naruto 16 10/10
Hinata 16 12/27
----------------------------------
Shino 16 01/23
Sakura 16 03/28
That seems to work, right? But what happens if I add an example year to the characters' birthdays?
Class Year 1 :: April 1 -- March 31
Neji 17 07/03 1989
Sai 17 11/25 1989
Lee 17 11/27 1989
----------------------------------
Tenten 17 03/09 1990
Class Year 2 :: April 1 -- March 31
Choji 16 05/01 1990
Kiba 16 07/07 1990
Sasuke 16 07/23 1990
Shikamaru 16 09/22 1990
Ino 16 09/23 1990
Naruto 16 10/10 1990
Hinata 16 12/27 1990
----------------------------------
Shino 16 01/23 1991
Sakura 16 03/28 1991
It would seem to work except for Tenten's birthday. If she was 17 on March 9 1990 to fit the Japanese school year model, then she would already have turned 18 before Sakura turned 16. But the databook says Tenten is 17 and Sakura is 16.
The only way I can see to make the ages, birthdates, and academy classes work out according to the databook is to use a calendar year.
Academy Class by Calendar Year
Tenten 17 03/09
Neji 17 07/03
Sai 17 11/25
Lee 17 11/27
----------------------------------
Shino 16 01/23
Sakura 16 03/28
Choji 16 05/01
Kiba 16 07/07
Sasuke 16 07/23
Shikamaru 16 09/22
Ino 16 09/23
Naruto 16 10/10
Hinata 16 12/27
So based on the latest databook, at least, Sakura is older than both Sasuke and Naruto. And if you think about it that makes sense. It not only makes Naruto "dead last" yet again in terms of Team 7, but the only person younger than him -- or more dead last among all the rookies and always trying to catch up to Naruto -- is Hinata.
ETA: Not to mention that using the calendar year is WAY easier for Kishimoto to have set up in the beginning and remember going forward.

Edited by KnS, 20 July 2013 - 10:53 PM.