I think there's a conflict in the ages, and it depends on which film you're trusting. If you follow the theatrical, its Ducky (we never see Petrie hatch so that's not debatable), but I think it's in LBT 5, Cera is claimed to be the oldest.
I feel most certain that Ducky is older than Cera in general. I feel that way because of the weather in the first movie. Right after Cera hatched, it started to rain, and did so for a while. Ducky, on the other hand, roamed about for a bit after hatching, and to the end of her scene, there is no rain.
As for the scene in the fifth movie, when both Cera and Littlefoot acknowledge her being the eldest, that may have certain significance to her “wanting to be the best” attitude that she portrays. First of all, there is the seniority rule, you know, where someone has or thinks to have seniority over those younger and is “higher“ than them therefore. Secondly, really little kids are more apt to believe things as they tend to have lesser mental guards that say, “That can’t be right,” or “I have my doubts about that.” Well, Cera flaunting the “fact” that she was older in the fifth movie certainly seems to portray that she does feel superiority with being the oldest. So, perhaps as previously mentioned here, Cera did lie to get the seniority status and thus be “better.”
It’s possible that even at a young age she felt the need to be the object of deference, and, being as young minds are more gullible, fathomed a very detailed and certain-sounding story that she is older than them and maybe even how she is as well. Having no reason to object, as the others don’t seem to worry about being better in any way, they agreed. Maybe something like, “Okay, sure. You’re the oldest. Thatës nice to know.” Now, nobody saw Petrie being born, so one may wonder why he never challenged the age issue. Well, that might be contributed to the fact that Petrie seems to be rather intimidated by her. I can list several instances if need be showing Petrie submit to her out of fear. So, maybe he never challenged her statement because he was afraid to challenge her.
The way I see it, this is how they are ranked in age, one being the oldest.
One- Petrie or Ducky
Two- Petrie, Ducky, or Cera
Three- Petrie, Cera, or Littlefoot
Four- Petrie or Littlefoot
Five- Spike
Still, my logic is usually illogical, so I wouldn’t take anything I wrote here too seriously. Later!