In this article, we suggest that aging and development are two sides of the same coin, and that developing a comprehensive understanding of health and disease risk requires examining age-related processes occurring throughout the earliest years of life. Compared to other periods in life, it is during this early period of acute vulnerability, when children's biological and regulatory systems are developing, that biological aging occurs most rapidly. We review theory and empirical research suggesting that processes of development and aging are intricately linked, and that early adversity may program biological parameters for accelerated aging and disease risk early in life, even though clinical signs of age-related disease onset may not be evident until many years later. Following from this, we make the case for widespread incorporation of biological aging constructs into child development research.