I'm not a fan of the way this prompt is worded for it suggests that a single event or accomplishment can be so transformative that one becomes an adult overnight. Maturity comes as the result of a long train of events and accomplishments (and failures). That said, this prompt is an excellent choice if you want to explore a single event or achievement that marked a clear milestone in your personal development. Be careful to avoid the "hero" essay — admissions offices are often overrun with essays about the season-winning touchdown or brilliant performance in the school play. These can certainly be fine topics for an essay, but make sure your essay is analyzing your personal growth process, not bragging about an accomplishment.