I have two daughters and one son. I don't remember for sure, but it was between 4 and 6 years old. Kinda depended on how well they listened and paid attention. I remember well that the smallest helmet wasn't small enough. Most of their riding experience was on a military installation at that age, so I felt fairly safe with the little ones on board. With the MP's crawling all over the place, there aren't too many cagers pulling stupid maneuvers. Longer hiway rides happened after age 10 or so when their heads grew into their helmets and I could count on the fact they were going to sit still and pay attention. Now they are in their late twenties and with the exception of my son (done riding passenger), my girls still love riding.
As far as precautions, their first rides were in front of me. Helmets, long sleeves and pants and little gloves. Slow riding, probably never over 30 mph. Avoiding busy traffic. Just used common sense and prayed for the best!