Well, this takes me into further speculation about the song during the ending credits of The Land Before Time II: The Great Valley Adventure. The scene shows several hatchlings moving about. Baby three horns, baby swimmers, even a baby spike tail.
What was with all that? Was it mating serason in the Great Valley? :huh:
Now, as they are showing these off, they show tiny flyers flying into the distance. They must be babies too, because why would they show babies of three species then show older flyers?
I think you even see them close up when the gang, plus the new hatchlings, run towards the camera at the end.
No baby longnecks though.

Thankfully we do see some in 9
The flyers must be babies too. Based off of this, I believe that flyers learn their aerial movement ability just shortly after hatching in the Land Before Time universe.
But we already know that from 1, when we see those baby pteranodons fighting over the cherry.