18 months & up. The smooth, pleasing texture of the River Rocks invites children to engage in one of the early STEM activities listed in the included set of activity cards. Children can stack, sort, and use the River Rocks in the water table and a variety of other areas in the learning environment. Children will build a strong foundation in basic science, technology, engineering, and math skills. Incorporating River Rocks into the learning environment is the perfect way to introduce STEM for toddlers! Largest river rock measures 4 1/2".