My son has been riding with me since he was 8, always ATGATT (big kid -- could reach the passenger floorboards on my K12LT at 7 1/2). Rode with me (spouse behind us on her R1150GS) from VA to Arkansas and back for a rally. The keys for us were an intercom so I could constantly check on his state, and the KLT's trunk and armrests -- between those and my back, he was physically locked in place. Made it through three Harry Potter books on CD by the time the trip was done.
I cannot over-emphasize how important the intercom was, both for safety and for permitting "bonding" conversations (as well as discussions of right/wrong driving examples we would see).
Here's the reason(s) we introduced him to riding young: I learned how to ride by buying a CB360T when I lived in the dorms at college in the '70's. I somehow survived riding it from the seller's house back to the dorm, and then proceeded to learn how to ride by somehow not plastering myself against the many cars/trucks/roadsigns/buildings in the area over the next year or two of running around campus or downstate a couple hours to my grandparents' on the weekends.
Fast forward a couple decades: Our son lives in a house with 5-6 motorcycles and two former MSF instructors as parents. There has never been any question that he would want to ride. So we started training him at a young age about ATGATT (great to hear a 9 yo over the intercom talking about how stupid the non-ATGATT t-shirt/shorts/sandals riders were
). We figured he stood a better chance of survival by living through several years of instruction under watchful eyes than by buying a bike when he's living at the dorms at college and ... (see above)
I have to say it's paid off. He's 18, a graduate of the MSF course, his learner's permit was recently replaced with an M class license, and I have to say he's safer and more awake/aware/thinking about "what ifs" than 90% of the riders you see out there on the weekend. I also believe it has been good for helping us get though the teen hormones/impending separation period with everyone's sanity basically intact -- gave us a common "safe" ground from which conversations could develop. Whether it contribued to the current straight A's and extraordinarily responsible behavior, I can't say; I doubt it hurt any, though.
Do I literally shudder at the thought of what could happen to him someday? You betcha; always will until they close my lid and light off the over-sized Bunsen burners. But I'd rather see him live life than watch others do it.