In metacognition research, performance judgements and their accuracy are considered pivotal for self-regulated learning and task performance. However, their long-term impact on academic achievement remains under-researched. This study investigated the role of performance judgements and their accuracy for later maths competencies and explored whether this relationship varied with age. We used data on student performance judgements in a maths test, actual performance, and performance in a maths test two years later collected from 5551 German primary and 4780 lower secondary school students. Response surface analyses supported none of the five competing hypotheses that we investigated. They indicated the dominant role of past competencies and a positive, although weaker, effect of judgements, especially at high competence levels. Students in both samples overestimated their performance, with secondary school students being more accurate. The study suggests refining theoretical models to better link past performance, performance judgements, and accuracy to short- and long-term achievement.