So this is sparked from Tumblr and my friend had something to puzzle out about the Inner Senshi's ages.  Supposedly, officially, Makoto is the 'youngest' of the group. (Manga wise because 90s anime won't give us anyone's birthday but Usagi's and only once there. )

She believed that Rei would've been because she started out as 13yrs old. 

I reasoned that Rei had her birthday first before the others and the order of birth for the reborn Inner senshi was: Rei, Usagi, Ami, Minako, Makoto.

But then I started thinking about which birthdays got representation in the manga.  We see Rei's is celebrated twice (Once in Casablanca which takes place during the DK arc and again with the Death Buster's arc).   Usagi/Chibiusa's once (Assumption is it takes place just after DB arc but before Dream arc.)   Ami's was once, (Takes place during Dream Arc.),  Minako and Makoto share their birthday party in December (Which is during the Kaguya story. Which we assume takes place before Dream arc.)

Just thought this could be an interesting discussion.  (and one that can wander off adding others to the Age list in order. )

Coincidentally, a recent Tuxedo Unmasked article suggests that it may be impossible to tell, since their birthdays and ages (at least in the manga where there's more info) suggest some of them are in the wrong grade (or perhaps their grade level suggests they're the wrong age). The contradictory information just makes it too hard to tell which is truth ;_;.

I haven't looked into it further than reading the article though, so if anyone finds other evidence that might help solve the puzzle that'd be pretty sweet XD.