Seabird conservation often focuses on nestlings and breeding adults. Yet imperiled seabird populations also contain prebreeders, including juveniles and subadults, that wait several years before breeding at colonies. We use previously-published data on reproductive and survival rates for 90 species to quantify the conservation relevance of prebreeding seabirds. We find, first, that prebreeders average about half of seabird populations (median 48.1%, range 7.6–70.0%). Second, while seabird population growth is much more sensitive to adult survival than prebreeder survival, human-driven changes may shift the importance of prebreeders for future population stability. Third, lowering the breeding age is a potentially useful, but underexplored, route to increasing population growth. Managing prebreeders could thus play a key role in protecting seabirds. This task may require answering fundamental questions about the behavior of young birds. Broadly, we suggest that life history traits (e.g., breeding age) shape both obstacles to, and opportunities for, successful conservation.