http://www.d20pfsrd.com/bestiary/monster-listings/dragons/dragon
Pathfinder conveniently unified the rules on true dragons, but they need a bit of adjustment for PC use. The following is based on "CR (+1) = ECL", working backwards from that.

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A dragon PC starts as a wyrmling of the chosen dragon species, using the stats in the Bestiary. His character level is equal to his racial Hit Dice (not his CR). He can increase in power by taking levels in the dragon racial class below:

Dragon Class
HD: d12
BAB: Good
Saves: All Good

Class Skills: Appraise, Bluff, Climb, Craft, Diplomacy, Fly, Heal, Intimidate, Knowledge (all), Linguistics, Perception, Sense Motive, Spellcraft, Stealth, Survival, Swim, and Use Magic Device.
Skill Ranks Per Level: 6 + Int

Lv Quote 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Powers Quote - Very Young - Young Juvenile - Young Adult Adult Mature Adult - Old Very Old Ancient Wyrm - Great Wyrm

Size Quote Base + 1 Base + 1 Base + 2 Base + 2 Base + 2 Base + 3 Base + 3 Base + 3 Base + 3 Base + 4 Base + 4 Base + 4 Base + 4 Base + 4 Base + 5 Base + 5

Natural Armor Quote Base +2 Base +3 Base +4 Base +6 Base +9 Base +10 Base +12 Base +15 Base +18 Base +19 Base +21 Base +24 Base +27 Base +30 Base +31 Base +33

Breath Weapon Quote Base x2 Base x2 Base x3 Base x3 Base x4 Base x5 Base x5 Base x6 Base x7 Base x8 Base x8 Base x9 Base x10 Base x11 Base x12 Base x12

Abilities Quote Str +2,  Dex -2, Con +2,  Int +0,  Wis +0,  Cha +0 Str +4,  Dex -2, Con +2,  Int +2,  Wis +2,  Cha +2 Str +6,  Dex -2, Con +2,  Int +2,  Wis +2,  Cha +2 Str +8,  Dex -2, Con +4,  Int +2,  Wis +2,  Cha +2 Str +10, Dex -2, Con +4,  Int +4,  Wis +4,  Cha +4 Str +12, Dex -2, Con +6,  Int +4,  Wis +4,  Cha +4 Str +12, Dex -4, Con +6,  Int +4,  Wis +4,  Cha +4 Str +14, Dex -4, Con +8,  Int +6,  Wis +6,  Cha +6 Str +16, Dex -4, Con +8,  Int +6,  Wis +6,  Cha +6 Str +18, Dex -4, Con +10, Int +6,  Wis +6,  Cha +6 Str +18, Dex -6, Con +10, Int +8,  Wis +8,  Cha +8 Str +20, Dex -6, Con +10, Int +8,  Wis +8,  Cha +8 Str +22, Dex -6, Con +12, Int +10, Wis +10, Cha +10 Str +24, Dex -8, Con +12, Int +10, Wis +10, Cha +10 Str +26, Dex -8, Con +14, Int +10, Wis +10, Cha +10 Str +26, Dex -8, Con +14, Int +12, Wis +12, Cha +12

Powers: The dragon receives the special abilities listed in its species entry for the given age category, and is considered to be that age category for all effects.

Size: The dragon's size increase to the given number of sizes above that of a wyrmling. Do not apply ability score or natural armor adjustments for increasing in size. The dragon's speed, number of attacks and breath weapon size increase as appropriate to his new size.

Breath Weapon: Multiply the wyrmling's breath weapon damage by the given amount to determine the dragon's breath weapon damage.

Natural Armor/Abilities: Modify the dragon's natural armor bonus to AC and ability scores by the given values. These modifiers are not cumulative.

Favored Classes
A dragon gaining a level in his favored class can select one of the following benefits in place of the extra hit point or skill point.

• Add +1/2 of a spell from the sorcerer spell list to his spells known for this class. This spell must be at least one level below the highest spell level the dragon can cast in this class.
• Add +1/2 to damage with any spell, spell-like or supernatural ability that deals damage of the same type as the dragon's breath weapon.