Despite being a talented athlete as a teenager, Caulker chose to pursue a career in football playing his early football with Hounslow Borough, his local club. After initially playing as a central midfielder, he acted on advice from a coach and made the switch to central defence. After a single match at centre back for Hounslow at the age of 15, he was offered trials with seven clubs including Tottenham Hotspur, Chelsea, Reading and Queens Park Rangers. Caulker joined the Tottenham Hotspur youth team and having broken into the Under-18s team as an Under-16 player during the 2007–08 season, Caulker progressed to make 31 appearances and scored one goal in 2008–09. He also forced his way into the reserve team that season, making five appearances, and captained the Under-18s towards the start of that season. He impressed during his first season as a full-time scholar and signed his first professional contract at Tottenham in July 2009.