Sullivan qualified for the 1995 and 1996 Under-21 World Championships. He finished third in 1995 at the Kaanaa Speedway Stadium in Tampere, Finland (won by Jason Crump). Sullivan finished in a 4-way tie on 12 points and defeated Tomáš Topinka (Poland), Kai Laukkanen (Finland) and Ben Howe (England) in a runoff to claim third place. In his final shot at winning the title before turning 22, he improved to second in 1996 behind Poland's Piotr Protasiewicz (who scored a 15-point maximum) at the Speedwaybahn in Olching, Germany. Sullivan defeated Danish rider Jesper B. Jensen in a runoff to finish second in 1996 after both finished the meeting on 11 points.