Just because he is more mature doesn't mean he is older. Besides, I'd say Ducky is the oldest because she is the first one we see hatch. Cera could have lied when she said she was the oldest. But maybe that wasn't Ducky we saw hatching. It may have just been some random swimmer.
I agree with you on this one, F-14.
Regarding the situation in LBT 5 where the Gang agrees that Cera is the oldest:
Let us remember a similar situation in LBT 7, where Cera is arguing about always being at the back of the line while crossing land gaps. The Gang agrees with Cera saying "she do have a point" for example, as if the Gang has forgotten the details of their past adventures. If we look back to earlier films, we clearly see that this whole story about Cera being last in line is complete bogus.
I'm saying, if such a false bit of information can be protrayed in LBT 7, why couldn't the same thing have occurred regarding the age discussion in LBT 5? I'm going with this:
The discussion in LBT 5 is false, incorrect information, as the series has a reputation for making such errors (sadly

).
As for who IS the oldest? The first LBT film shows a swimmer being born, who is
presumably Ducky. The fact is, we don't know for sure whether this is Ducky; this scene may be interpretted in a way to show various families bearing children, of all different walks of life, not necessarily ever destined to work together. This shows the vastness of the species headed away from this land, to the Great Valley (or other hospitable land).
It is probably a safe assumption that our little swimmer from the first scene is Ducky; even her first word, "Momma", is spoken in a typical Ducky voice. However if we look towards the later sequels, namely LBT 8, we see that Ducky's voice is very, Very similar to her siblings. I have reason to believe that Aria's quotes were reused for the voices of Ducky's siblings in LBT 8. My point? Perhaps all baby swimmers sound so similar that telling them apart by voice may be nearly impossible. We cannot Prove that this is Ducky in the first scene, I don't think. If someone can correct me, that would be fantastic. B)
For all intents and purposes, everyone in the Gang is the same age, except our friend Spike. However when it comes down to technicalities....
Spike is clearly the youngest, as the rest of the Gang finds him when he's still an egg.
Petrie is unable to fly when the gang meets up with him, therefore I believe him to be one of the younger members of the Gang.
Ducky is a fairly capable swimmer, therefore she can't be especially young (can she?) I'm saying she's considerably older than Spike. However she has grown very little (compare her size to the size of one of her eggs in LBT 2. She has hardly grown at all).
Littlefoot and Cera both grew a substantial amount from when they were first seen, therefore, I put them among the older members of the Gang. Unless the discussion in LBT 5 is indeed true, we have no way of figuring out which of these two is older, I don't think.
Based on this information, I'm saying, from Oldest to Youngest:
Littlefoot/Cera
Ducky
Petrie
Spike