Koen was born in Bloemfontein, but grew up in the North West province. He went to Hoërskool Klerksdorp and was selected to represent the at the Under-16 Grant Khomo Week in 2010, scoring tries in their matches against Griquas and Border Country Districts. In 2011, he was named in the Leopards team that competed at the foremost rugby union competition at high school level in South Africa, the Under-18 Craven Week. He again scored two tries during the tournament, both in a 17–40 defeat to Eastern Province Country Districts.
2013 : Leopards Under-19
In 2013, Koen was included in the squad that participated in the Under-19 Provincial Championship. He started eleven of the team's twelve matches during the regular season, scoring tries in matches against and as the team finished on fourth position on the log to qualify for the semi-finals. Koen also started the semi-final against, but the team from Pretoria won 37–21 to end the Leopards' interest in the competition.
For the 2014 season, Koen moved to Durban to join the academy. He started nine of the side's twelve matches in the 2014 Under-21 Provincial Championship, scoring a try against his former side the Leopards in a 55–41 victory and a second in a 106–3 win over. The Sharks finished in fifth spot, failing to secure a semi-final berth. At the start of 2015, Koen was included in the squad for the Vodacom Cup competition. He made his first class debut by coming on in the 28th minute of their match against the, helping his side to a 53–0 win. After two more appearances as a replacement against and the, he made his first start in a 24–27 defeat to in Pietermaritzburg, before reverting to the bench for their final match against the. He made four appearances for the during the Under-21 Provincial Championship in the latter half of 2015, again scoring a try against his old side the Leopards in a 45–12 win.
2016–present : Griquas
In 2016, Koen joined. He made eleven appearances for the Kimberley-based side during the 2016 Currie Cup qualification, scoring his first senior try in their 36–14 victory over a, as Griquas won eleven of their fourteen matches to finish the competition in second place to earn a spot in the Premier Division of the Currie Cup. He made his debut in the Currie Cup's Premier Division as a replacement in their first match of the season – against former side the – before making his first start a week later in a 30–24 victory over defending champions the.
