Over the past decade, online learning has witnessed tremendous growth in popularity due to its ability to reach diverse participants in a scalable manner. However, one primary area of concern is the low course completion rates in digital platform-based learning, compared to face-to-face counterparts. Given that most education tends to be organized by age, we ask: how does the degree of age-similarity among cohort peers affect course engagement and persistence? Using a unique dataset of 17,000 working professionals enrolled in business skills training courses offered by an elite U.S. business school over a three year period, we show that age similarity has a positive effect on individual course completion: an individual’s likelihood of course completion increases by 3% for every 10 same-age cohort peers. Given that the average cohort size is 220 people, this suggests that a small threshold of same-age peers can have a substantial impact on course engagement and persistence. To examine mechanisms, we turn to participants’ motivations for taking the course, and find that similar-age peers are more likely to affiliate with one another because they share a common motivation for taking the course. Our results suggest that there is an implicit trade-off between social engagement and diversity of perspectives in online courses, and that the organization and structure of online courses ought to balance both objectives.