The standard advancement rules allow dragons theoretically infinite progression even beyond the stage of great wyrm. The following rules let dragons improve more than their Hit Dice as they progress to epic heights of power.
Age Category: A standard dragon gains one ?virtual age category? for every 3 Hit Dice it gains beyond the great wyrm stage. Abilities that function once per day per age category or otherwise use the dragon?s age category as part of a calculation use this adjusted number. Epic dragons gain one age category per 5 Hit Dice beyond great wyrm.
Size: One important element of dragon advancement is increasing size. The dragons that don?t reach Colossal size by the great wyrm stage can never reach it according to the standard advancement rules. When advancing a dragon, consider its basic size group: lesser (white, black, brass, and copper dragons), ordinary (green, blue, and bronze dragons), greater (silver, red, and gold), or epic (force and prismatic). A dragon that is Tiny as a wyrmling is in the lesser group, a dragon that is Small as a wyrmling and never reaches Colossal size is in the ordinary group, and a dragon that is Small to Large as a wyrmling and reaches Colossal by the great wyrm stage is in the greater group. A lesser dragon becomes Colossal when it gains two age categories (6 Hit Dice) beyond great wyrm. It increases to Colossal+ when it gains an additional four age categories (12 HD). An ordinary dragon becomes Colossal when it gains one age category (3 Hit Dice) beyond great wyrm. It increases to Colossal+ when it gains an additional four age categories (12 HD). A greater dragon becomes Colossal+ when it gains four age categories (12 HD) more than it needed to reach the Colossal size.
Epic dragons, already Colossal+ by the time they reach the great wyrm stage, do not increase in size thereafter. Use the information presented under the epic dragon entry, below, to determine the statistics of a Colossal+ dragon.
Armor Class: A dragon?s natural armor bonus increases by +1 for every Hit Die it gains beyond the great wyrm stage. (This rule applies for lesser dragon advancement as well, since natural armor and Hit Dice always increase at the same rate.)
Breath Weapon: If a dragon?s breath weapon deals damage, the damage typically increases by 2 dice for every virtual age category the dragon gains. The two exceptions among standard dragons are the brass and white dragons, whose breath weapon damage increases by only 1 die per age category. The saving throw DC against a dragon?s breath weapon remains 10 + 1/2 the dragon?s Hit Dice + its Consitution modifier.
Spell Resistance: For standard advanced dragons, spell resistance increases by 2 per additional age category. For epic dragons, spell resistance increases by 3 (for force dragons) or 6 (for prismatic dragons) per additional age category.
Move: When a dragon becomes Colossal, its fly speed increases by 50 feet and its maneuverability becomes clumsy. When it reaches Colossal+, its fly speed increases by another 50 feet, and its maneuverability remains clumsy. A dragon?s land speed and other special movement s (swim, burrow, and so on) do not change.
Ability Scores: A great wyrm?s Strength and Consitution scores both increase by +2 for every virtual age category the dragon gains. Its Dexterity remains unchanged. Its Intelligence, Wisdom, and Charisma increase by +2 for every two age categories the dragon gains.
Special: Dragons do not gain additional spell-like abilities, but their damage reduction continues to increase as they gain Hit Dice. For most dragons (the exceptions being the force and prismatic dragons), the great wyrm?s damage reduction increases to 15/epic after exceeding the great wyrm age category and the dragon?s natural weapons count as epic weapons for the purposes of bypassing damage reduction.
Force and prismatic dragons have the following DR progression: wyrmling 10/magic, young 15/magic, young adult 15/epic, mature adult 20/epic, very old 25/epic.
Caster Level: A great wyrm?s caster level increases by 2 for every virtual age category the dragon gains. As with the epic dragons described later, advanced dragons gain the Improved Spell Capacity feat as a bonus feat once for every three caster levels above 20th.
Feats: Like ordinary dragons, advanced dragons receive one feat for every 3 Hit Dice they have. Feats gained after the dragon attains great wyrm status can be epic feats.
Challenge Rating: For standard dragons, CR increases by 2 per additional age category. For epic dragons, CR increases by 3 per additional age category. All other dragon statistics are as presented for dragons in general and specific dragon varieties.
SEE WIKIPEDIA ENTRY: Dragon

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System Reference Document -> Epic Creatures (SRD)