Koster was educated at St. Andrew's College, Grahamstown, and played rugby for the school. For his performances during Craven Week, he was called up to play for Eastern Province Elephants to play for their Country Districts team in 2017 but left them to join the Sharks youth system in 2018. In the same year, he played for the Sharks' Currie Cup team when they won the 2018 Under-19 Provincial Championship against the Blue Bulls. In 2019, Koster played for the Sharks XV in the Rugby Challenge. Later in the year, he was invited to a South Africa national under-20 rugby union team selection camp for the Junior Springboks in the 2019 World Rugby Under 20 Championship, however he was not selected. In 2020, he was added to the Sharks' Super Rugby squad. However, a few months later, the season was cancelled due to the COVID-19 pandemic. The teams from South Africa instead played in the Super Rugby Unlocked tournament over October and November.