Assessment systems that track children's progress across time need to be sensitive to the variegated nature of development. Although instruments are commonly designed to assess behaviors within a specific age range, some children advance slower or faster than others and, as a result, often show behaviors from a younger or older age group. This situation is particularly common when the assessment is carried out close to the intersection of two age groups (e.g., just before preschoolers enter elementary school). Assessments that measure development across multiple age groups need to be able to capture behaviors across these cross-age transitions to make valid inferences about children. In other words, one needs to evaluate the extent to which constructs are measured reliably and without discontinuities across the whole age range. This article describes a process for establishing validity of cross-age inferences by using a set of age-consecutive developmental assessments. Identifying and validating an alignment structure between instruments is an essential component of the assessment development process. We present the methodology for establishing cross-age validity using the Desired Results Developmental Profile-Revised (DRDP-R). The DRDP-R assessment consists of a set of instruments designed to track children's socio-emotional, cognitive, physical, and behavioral development across three age groups from kindergarten through 12 years old. We discuss the principles, methodology, and challenges involved in constructing such complex systems of developmental assessments. (Contains 3 tables, 6 figures and 7 footnotes.)