Scientific reasoning refers to the thinking skills involved in conceiving and conducting an investigation. This study examined how proficiency in performing these skills develops during the upper-elementary school years. A sample of 157 children (age 7–10) took a performance-based scientific reasoning test in three consecutive years. Four distinct developmental patterns emerged from their annual test scores, which were independent of prior domain knowledge and sociodemographic characteristics except gender. Developmental patterns in scientific reasoning and reading comprehension, but not math, were related such that many children with a high entry level or accelerated growth in scientific reasoning also performed better and progressed more in reading comprehension. These results indicate that scientific reasoning develops differently in same-age children, largely independent of personal characteristics but generally comparable with reading comprehension.