Wyrmlings, how high can they fly?

My players managed to make an alliance with a pride of wyrmlings. They convinced them that it is fun to carry people around and play chase if anyone ever falls out of the skies.

I know they’re going to try to ride those creatures for sure and I want them to have their fun. But just in…RAW does not stipulate the maximum height flying creatures can fly, but there should be a limit, at least I feel like?

How high can dragons and their younger counterpart be flying?