As a junior curler, Mosaner represented Italy in four World Junior Curling Championships. He played third for Italy at the 2012 World Junior Curling Championships, on a team skipped by Andrea Pilzer. The team finished 9th. Mosaner then became skip of the Italian junior team for the next three World Juniors. At the 2013 World Junior Curling Championships, Mosaner led his team of Pilzer, Daniele Ferrazza and Roberto Arman to a 6th-place finish (5-4 record). At the 2014 World Junior Curling Championships, he led his team of Sebastiano Arman, Ferrazza and Roberto Arman to a 5th-place finish, losing in a tie-breaker to Switzerland. Finally, at the 2015 World Junior Curling Championships, he was less successful, leading his team of Sebastiano Arman, Carlo Gottardi and Fabio Ribotta to an 8th-place finish (3-6).