Negative age stereotypes about older employees are present across industries and they are particularly strong in technology-related jobs. They can hinder cooperation and team processes, which are of utmost importance in software development. This paper proposes and compares two interventions to reduce age stereotypes in software development. An awareness-based intervention was conducted on-the job, as a quasi-experiment with 56 participants. A cooperation-based workshop was conducted as a field experiment with 74 employees. Both types of interventions reduced bias in performance and innovation expectations favoring middle-aged over older employees. The reduction in biases held by developers was particularly strong. Only the cooperation-based intervention reduced bias toward both older and younger employees. This intervention led to a long-term (six months) reduction in bias, regarding developer performance expectations. The study extends the diversity training literature in establishing causal, long-term effects for age stereotype reduction in the field. Furthermore, it contributes to the literature by indicating that contact hypoth ... mehresis can not only be applied to reduce age stereotypes toward older but also toward younger employees. The design enables practitioners to create on-the-job diversity interventions that employees are willing to attend, and, thus, to reach a majority of employees without interventions being mandatory.