Koen was born in Bloemfontein, but grew up in the North West province. He went to Hoërskool Klerksdorp and was selected to represent the Leopards at the Under-16 Grant Khomo Week in 2010,[2] scoring tries in their matches against Griquas[3] and Border Country Districts.[4]
In 2011, Koen 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.[5]
In 2013, Koen was included in the Leopards U19 squad that participated in the Under-19 Provincial Championship.[6] He started eleven of the team's twelve matches during the regular season, scoring tries in matches against Border U19[7] and Free State U19[8] as the team finished on fourth position on the log[9] to qualify for the semi-finals. Koen also started the semi-final against Blue Bulls U21, but the team from Pretoria won 37–21 to end the Leopards' interest in the competition.[10]
For the 2014 season, Koen moved to Durban to join the Sharks academy. He started nine of the Sharks U21 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[11] and a second in a 106–3 win over Border U21.[12] The Sharks finished in fifth spot, failing to secure a semi-final berth.[13]
Koen made four appearances for the Sharks U21 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.[19]
In 2016, Koen joined Griquas. 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 Sharks XV,[20] as Griquas won eleven of their fourteen matches to finish the competition in second place[21] 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 Sharks[22] – before making his first start a week later in a 30–24 victory over defending champions the Golden Lions.[23]
