In 2013, Kirsten was selected in the South Africa Under-20 team that attempted to retain the IRB Junior World Championship title that they won in 2012. Kirsten didn't feature in their opening match of the competition as they beat the United States 97–0. He was named on the bench for their next match against England, but failed to make an appearance in their 31–24 victory. He did, however, feature in their final pool match against hosts France, coming on as a replacement for the final ten minutes of a 26–19 victory which ensured South Africa finished top of their pool and qualified to the semi-finals. However, that proved to be Kirsten's only involvement in the tournament, as he sustained a fractured cheekbone during a training session, which ruled him out of the remainder of the tournament. In his absence, South Africa lost their semi-final match 17–18 to Wales before clinching third spot in the competition by beating New Zealand 41–34 to win their fourth third-place play-off match in six years.