My only expereince was with two warbird riders many years ago. Both about 13-14 and they were getting a T-34 ride. One kid was an obvious "no" from the moment he got out of his parents car. "Busy" did not begin to explain this kid. No way was he going to sit in the rear seat of anything without touching, pulling, pushing anything within reach. The pilot said no and we totally backed him up. Good fortune had a flight school and a cessna who, where the kids Dad could ride shotgun with him, was happy to take him for a ride.
The other kid was awesome. He listened, even asked a few good questions and from just watching him for 15-20 minutes you could tell what kind of person he was. The ride went great and I believe he is a commercially rated pilot today (I'd like to think because of that ride).
The point is that based on several factors (pilots good judgement, third party support (no pressure to do the ride), observation of passenger) that the risk factor was reduced to an acceptable level. I do not think that under 10 is a good plan in a situation where rear seat egrees would be tough, but then I am not a pilot of a Mustang, T-6, Corsair etc. Hopefully one day I will have to make this decision for myself!
One last observation - there are many graves attesting to the difficulty of getting out of a large airplane (B-24, B-29, B-17) during an emergency situation by well trained and expereinced crew members. Actually, thinking about it, I know of a situation that ended badly where the pilot did not want to leave a passenger stuck in the belly seat of a plane, in this case a Skyraider.
Tom P.