Juan Mostert represented Western Province at the Grant Khomo week in 2016 as well as Western Province u/18 Craven Week in 2018. After a very impressive Craven Week Juan was Selected for the 2018 u/18 SA Schools side.
At flyhalf, Juan Mostert was especially important to the hopes of the Paul Roos 1st XV of 2018. He ensured that the lethal trio of centre Brendan Venter, leftwing Remy Engelbrecht and rightwing Regan van der Westhuizen received the possession required to unleash their attacking capabilities on their opponents. During the Craven Week, pivot Juan Mostert pulled the strings, with the Western Province backline operating like a well-tuned engine. Juan continued his form into the u/18 International series and was sorely missed against Grey College with many blaming the loss against Grey College due to the maestro of the backline, Mostert absence.