Jeff Carlsen
Adventurer
The lower level dragons' stats have a use, so they should be in the Monster Manual. They're vaguely playable as PCs (In PF, at least, due to its highly generous monstrous character rules, less so in 3.x), shapechange forms, effigies, illusions, summons, or cohorts. But they shouldn't appear in published adventures or random encounter tables, due to the aforementioned "killing baby dragons" issue. Except maybe in a "dragon parent protecting its young from adventurers" capacity.
Perhaps, but I see younger dragons as something more befitting the Draconomicon.