Reflective Carapace. Any time the tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 6, the adolecent tarrasque is unaffected. 

Siege Monster. The adolecent tarrasque deals double damage to objects and structures.

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., single target. Hit: 12 (2d8 + 5) [slashing] damage. 

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., single target. Hit: 13 (1d12 + 7) [Piercing] damage.  If the target is a creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the tarrasque can’t bite or attack another target.

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., single target. Hit: 11 (2d6 + 5) [Bludgeoning] damage.

Horns. Melee Weapon Attack: +7 to hit, reach 10ft., single target. Hit: 14(2d10 + 4) [Piercing] damage.