Sebastian was a consolation and bonus finalist at the 2007 EISL Championships. He placed 19th in the 1000 yard freestyle, 21st in the 500 yard freestyle, and 14th in the 200 yard freestyle. He looks forward to swimming middle-distance freestyle events while representing the Yale Bulldogs in the 2007-2008 season. Sebastian started swimming at age six with the Smokey Row Swim Team. He swam for the Carmel Swim Club for eight years and represented the Carmel High School Greyhounds as a varsity swimmer for four seasons. Sebastian earned a perfect 100 points at the Indiana Age Groups State Championships both at age twelve and age fourteen. As a junior at Carmel High School in 2004, Sebastian helped his team win a fourth consecutive Indiana High School State title with victories in the 500 yard freestyle, 200 yard freestyle relay, and a third place finish in the 200 yard freestyle. The Greyhounds went on to place first nationally in all three relays and win the National Mythical Title that year for public and private schools. As a senior at Carmel High School, Sebastian placed third in the 500 yard freestyle and sixth in the 200 yard freestyle at the Indiana High School State Championships. In addition, Sebastian placed third in the 400 yard individual medley at the Indiana Senior State Championships held that year. In the summer of 2006, Sebastian competed at the Speedo Junior National Championships in Irvine, California. At Carmel High School, Sebastian was a member of the National Honor Society and a National Merit Finalist. He was a co-founder and co-president of the Buffet Club, Carmel's first club dedicated to eating.