Mine started at about 6 but that was due to lack of funds on my part rather than their age.
Make sure you've locked the doors! My inquisitive son decided to undo his belt and lean over from the back seat onto the door handle of a c172 whilst I was in a steep bank. I would have hated to explain how he had fallen out if the door wasn't locked. If they are in the front and not old enough to appreciate the consequences of their actions make sure their seats are too far back to allow them to grab the controls. Took me by surprise when a friend's son (who I thought was sensible) grabbed at the yoke when I was climbing out....
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When the wheels stop turning you're high enuff. CubTrek. To slowly go...