Artemis Fowl and the Eternity Code by Eoin Colter Book 3

Artemis Fowl is going straight-as soon as he pulls off the most brilliant criminal feat of his career... but his plan goes awry, leaving his loyal bodyguard, Butler, mortally injured. Artemis knows his only hope to save Butler lies in fairy magic, so once again he is forced to contact his old rival, Captain Holly Short of the LEPrepcon fairy police. Miraculously, Butler is healed, but there is a catch: he has aged fifteen years.