Threshold concepts are essential ideas about the natural world that present either a barrier or a gateway to a deep understanding of science. Weight, mass, and gravity are threshold concepts that underpin students' abilities to understand important ideas in all fields of science, embodied in the performance expectations in the Next Generation Science Standards (NGSS). This study begins with a review of research on students' difficulties in understanding these concepts individually and in relation to each other, based on individual interviews and surveys of several hundred children that illustrate how students' understanding of weight, mass, and gravity develops over the lifespan, from age five through adult. New data from an additional 451 subjects in the critical age range of 10 to 14 years old support and extend the prior findings. The purpose of the current study is to provide teachers and curriculum developers with actionable and up-to-date information that educators can use to help children at the upper elementary, middle, and high school levels achieve Next Generation Science Standards.