Youngest I ever had on my bike was the 14 yo son of one of my girlfirends. He wanted to go to the motocross races I was working and all I had at the time was my FZ750.
He did well except for one thing..on the trip to the track very early in the morning, he kept falling asleep! Had to pull over a few times to lecture him about staying awake.
I feel that 14 is the youngest kid who should be a pax on a bike..that is about the lower limit of the ability to follow instructions and also have enough physical strength to hang on.