We practiced the RIE principles from birth. My daughter was always free to explore and was trusted in her self-initiated movement. When she was about 15 months old, we were at a playground with a tall structure. She climbed the stairs to the structure and I stayed on the ground. She came to an open section with a drop off. I told her there was an edge and a drop off. She came to the side peering over. Then she held onto a bar dipping her one foot over the edge bending her knee to test the depth of the drop. She did this over and over until she seemed satisfied with the fact that the drop off was deeper than she could reach with her foot. She never once came close to falling. She knew where the edge was and trusted herself.
Principle: Freedom to explore