At a young age, Allen considered a career in football, having trials with Sheffield Wednesday and Nottingham Forest before concentrating on snooker. Playing out of the Fountain Club in Antrim, he was Northern Ireland U14 champion in 2000, the U16 champion in 2001, before winning the Irish U16, U18 and U19 titles in one weekend in 2002, becoming the first player to win all three tournaments. By the age of 16 he had recorded his first maximum break and was a winner of the Golden Waistcoat, a tournament for the best U19 players from around the world.