<shudders>
As long as this one is the exception, not the rule, I'll live with it.
Okay then. Using the Advanced Young notes we've got, I'll set the abilitie scores halfway between the baby and the adult's and plug in some appropriate feats.
How about this?:
Advanced Young
When a young gargantua advances to Gargantuan size, its statistics improve to the following:
Juvenile Reptilian Gargantua
Gargantuan Dragon
Hit Dice: 32d12+320 (464)
Initiative: +5
Speed: 40 ft. (8 squares), swim 30 ft.
Armor Class: 39 (–4 size, +1 Dex, +32 natural), touch 7, flat-footed 38
Base Attack/Grapple: +32/+60
Attack: Claw +44 melee (2d10+16/19-20)
Full Attack: 2 claws +44 melee (2d10+16/19-20) and bite +43 melee (4d10+8) and tail slap +42 melee (5d12+24)
Space/Reach: 20 ft./15 ft. (30 ft. w/tail)
Special Attacks: Crush 6d12+24, frightful presence, improved grab, swallow whole, thunderous roar, tail sweep 5d10+24, trample 6d10+16
Special Qualities: Damage reduction 10/epic, darkvision 60 ft., empathic sense, fast healing 15, immunity to sleep and paralysis, light sensitivity, low-light vision, water breathing
Saves: Fort +28, Ref +19, Will +20
Abilities: Str 43, Dex 13, Con 31, Int 6, Wis 14, Cha 20
Skills: Jump +55, Listen +37, Spot +37, Survival +37, Swim +24
Feats: Awesome Blow, Blind-Fight, Dire Charge, Improved Bull Rush, Improved Critical (claws), Improved Initiative, Multiattack, Power Attack, Snatch, Track, Weapon Focus (bite)
Environment: Any coastal land or aquatic
Organization: Solitary or family (1 plus 2 adults)?
Challenge Rating: 18?
Treasure: None
Alignment: Always chaotic neutral
Advancement: 33-63 HD (Gargantuan)
Level Adjustment: —