None of the Gang really knew exactly when they hatched compared to each other (besides Spike of course), so my guess is that Cera just speculated that she was the oldest, thinking that she was better and more grown up than the others. However, Ducky was shown to be hatched first in the first film, followed by Cera and then Littlefoot as the second-youngest, possibly. But, who knows? Petrie may have been the first to hatch, or anywhere in between, since we never see him hatch at all. I'm also going to go with Ducky being the oldest and Cera just speculating, since the gang probably has no idea of the exact day they were born.