Why not just make the dragon an age category older, but keep it the same Large size (like, it's body hasn't quite matured as rapidly as its other capabilities) - everything is the same, except the reach, and it gets the needed bump in CR. I think going from Juvenile (Large) as a red dragon to Young Adult (Huge) gives it a bump of 3 in CR. Probably more effective than 3 levels in a class, too. And, it will keep your players from subconsciously metagaming - "I see a Large miniature, so that means its CR:10 and its breath weapon does 8d10!" But, when you roll 10 dice instead of 8, it will give them pause.