This is likely to take several articles, as the first one or two are going to be somewhat vague as I try to find the shape of things.
This is in part because they appear to be under-CRed compared to other monster types. Draconic Hit Dice are better than most Hit dice granted by monster type (d12 hit points, full Base Attack Bonus, best saves across the board, and dragons have high natural armor, lots of immunities, and often have a bunch of spell-like abilities – and a powerful breath weapon), and at that they have high ratio of Hit Dice to Challenge Rating. Anything I do that brings them closer to balance with the rest of the monsters is likely going to look like large nerfs rather than minor adjustments.
I think it’s worth the effort. I’d rather see a weaker “CR 13” young adult red dragon that actually is a reasonable challenge at 13th level and knowingly choose an adult (CR 15) or mature adult (CR 18) dragon to make for a tough fight than pick a mislabelled CR 13 monster knowing that the label is incorrect.
This looks like it’ll be a long job and take a few articles, starting soon.