Diversity, equity and inclusion (DEI) training can benefit STEM students. However, typical college settings often limit college students' exposure to adult peers with intellectual and developmental disabilities (IDDs), a historically marginalized group. To lower this barrier, we developed a continuing education program, Lifelong Learning with Friends (LLWF), aimed at adults with IDDs on a large university campus, which provides diversity training to college students. In this program, undergraduate and graduate students from scientific and education disciplines are recruited to volunteer as peers and helpers. LLWF has reached hundreds of students with and without IDDs each year and more than 1,500 over the past 12 years. In our program, college students gain DEI training through learning sophisticated academic topics, including sciences, alongside adults with IDDs. Almost half (42%) of surveyed LLWF college volunteers did not have prior exposure to people with IDDs. Following program participation, we found that, irrespective of prior exposure, nearly all (98%) of volunteers had elevated their expectations of people with IDDs and reported increased interest in IDDs-focused research, education, social interaction, and advocacy. Additionally, college volunteers reported that they improved their science communication by seeing how science could be taught to a broad audience that includes adults with IDDs. We therefore suggest that other universities may consider our LLWF model to enhance DEI training by expanding opportunities for neurotypical students to befriend and learn science alongside adults with IDDs.