Getting to know the kids is the most important criteria and age after that. I have seen very young children that are more trustworthy than older kids. As far as outside goes in a fenced yard a child over age 3 should be ok provided I am able to watch them from in the house more or less continuously such as playing in the sandbox while I make lunch. For inside it is more about skill. A child that has proved they can safely climb onto the sofa and get down and always sits when they are on it can be trusted for a few minutes while I am in the next room/bathroom. If not they go into a playpen or come with me.