The Defining Decade by Meg Jay (Review & Book Notes)The Defining Decade by Meg Jay argues that the twenties are the most crucial and defining decade of adulthood, a time of significant brain development and opportunity for adults to make decisions about work, relationships, and identity. The book provides practical guidance and uses real-life stories to help young adults proactively build the lives they want, counteracting the cultural message that this period is an extended adolescence. Jay emphasizes the importance of being intentional about choices, building “identity capital,” and understanding that not making decisions is a decision in itself.