Originally Posted by Mpray1983
Jamie asked to be in the kingsguard so he could remain close to Cersei and avoid a marriage with Lysa Tully. Aerys agreed and made him a whitecloak despite his young age of 15 which was unheard of only because of the opportunity to dick over Tywin and rob him of his heir.
Actually, a bit of both:
At the age of fifteen, Jaime was knighted on the battlefield by Ser Arthur Dayne during the campaign against the Kingswood Brotherhood, during which he saved Lord Crakehall from Big Belly Ben and crossed swords with the psychotic Smiling Knight. On his return to King's Landing he stopped to visit his sister, from whom he had been separated for years. It was there that Cersei told him that Lord Tywin intended to marry him to Lysa Tully, second daughter of Lord Hoster Tully of Riverrun. Cersei suggested that Jaime become a member of the Kingsguard instead, replacing the recently deceased Ser Harlan Grandison, to be close to her and free himself of the unwanted marriage to Lysa.
After a night of passionate sex he gave his consent to Cersei's plan and within a month she has orchestrated it.
Jaime Lannister portrayed by Nikolaj Coster-Waldau in the HBO series.
A moon's turn later, Jaime was raised to the Kingsguard by Lord Commander Ser Gerold Hightower in a ceremony during the Tourney at Harrenhal, making him the youngest Knight to ever to be raised to the order. That night, however, King Aerys soured the honor by sending him back to King's Landing to guard Queen Rhaella and Prince Viserys, depriving him of the chance to participate in Lord Whent's tourney. Jaime realized then that the King had only chosen him for the Kingsguard as a slight against Lord Tywin, of whom Aerys was insanely jealous, to rob him of his heir.
 Furthermore, the plan failed to bring Jaime and Cersei closer together, as they had intended, when Lord Tywin, furious at the turn of events, resigned the Handship and returned with Cersei to Casterly Rock. Jaime remained at court, guarding the King.http://awoiaf.westeros.org/index.php/Jaime_Lannister