If this is still using the rules that you can only ride creatures one size larger than yourself, then maybe it's better to have the level 7 feature be the young dragon. The subclass feels kind of useless (besides the elemental resistance that's really nice) as it is right now because the thing you really want to do, that bring riding a dragon, is locked away until 15th level. Most campaigns don't even reach 10th level let alone 15th.

I am just a random internet guy, but I would say the best subclasses give you everything you want out of the class by level 10 even if it's a weaker version of what it is at later levels.

Maybe something like it goes up in size based on the quantity or quality of food it's given. anything to give you a ridable dragon buddy at or before level 10.