16-18 is perfectly cool...Lightning is the youngest, I know I'm making the fire dragon 26, so its completely up to you...but it could be cool, like lightning is in the middle of a high school hallway and starting to shift, and none of the other dragons are around to help out.
On that note another thing I forgot to mention. When the dragons are fully shifted, they can communicate telepathically. When they are in their human forms, they can only feel the presence of the others, within a certain distance, undetermined yet.
And if any of you guys have ideas feel free to throw them out here and we can see what we can work with