I let my oldest play alone in the fenced backyard, with the door to the house open, at 2.5yo, although I was nervous about it. I let her play alone in our unfenced backyard at age 5 or 6. I let the younger ones play with her in the unfenced backyard when I trusted them to stay in the yard (probably age 3 or 4). For some reason, neither my middle nor my youngest will play alone outside, or without someone older (in the case of the middle), even though I have no problems letting them
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