The possibility for age discrimination and stereotypes to affect performance evaluations is rising. Although careful evaluations might be expected from conscientious raters, little is known about whether they might show more or less bias towards certain age groups. Therefore, in our study using a time-lagged design, we investigated the effects of rater conscientiousness on the performance evaluations of younger and older actual co-worker (N = 242). We found that raters who were more conscientious provided higher ratings for older workers than for younger workers on task performance and organizational citizenship behaviours. Specifically, we tested the model of mediated moderation, in which the relation between rater conscientiousness and ratee age predicts ratee-perceived conscientiousness, which in turn predicts performance ratings. The model was significant for older ratees, but not for younger ratees. We discuss our results in terms of the “similar to me” effects and implications for organizational practices.