Large amounts of HD are basically one of the special qualities of a dragon. They rely on those big piles of dice and base bonuses to make them a challenge for their intended CR. If I were writing a dragon class one of it's main features would be getting more HD than its level would normally indicate. Counting each HD as, say, 3/4 of a level.

In the context of this thread, that would be essentially a negative LA. Naturally the wyrmling is the most playable, but how are you going to play them? Sure, you've got some movement modes and some stats, a little armor and a mild breath weapon. At this level the value of flight starts to fall off - more and more opponents are going to have a response to someone trying to flap around just out of reach. You're fast, sure, but running away doesn't win a fight. Maybe you can build yourself for strafing. Just never go indoors.

Your fighter buddy may not have wings (unless he bought some) but his full plate is comparable to your armor. He may not have your stats (although he might for the important ones if he's a monster race or templated). He can use a bow. All you've got is a pile of numbers that, sure, they're nice. But they aren't class features.

Dragons of all sorts don't get anything particularly interesting until the HD is already approaching the epic boundary. The wyrmling can have a +0, cuz they still have room to try to build something. But beyond that I'm reasonably comfortable giving every dragon of every type (except steel or anything really weird I'm not aware of) a -0 for every age category past the first. Dragons are HD bloated - it's one of their defining features.