The field of youth development is based on a long tradition of youth work practice, a body of research on adolescent development, an increasing understanding of developmental processes, and work in related applied fields like education and child care. Whereas multiple lists have emerged describing what comprises or is needed for youth development, the field lacks a dynamic and appropriately complex theory or paradigm that captures what is known and stimulates new insights. A paradigm shift that incorporates changes in basic assumptions and a new understanding of the dynamics involved is critical to examining how after-school and other nonformal learning programs can meet the needs of youth in the middle years (ages nine to fifteen). In this article, the author articulates six fundamental tenets of youth development, proposes an analogy to help elucidate the dynamic dimensions of the field, and then explores the implications of this analogy for creating a new paradigm. The tenets, when combined and examined in light of a new analogy built around parallels to the elements of diet and exercise, can provide insight into the importance of out-of-school-time opportunities for learning and development, especially during the middle years. (Contains 1 table and 11 notes.)