Twelve is much too young to be driving a car in a town full of people and other vehicles, but if this happened on some isolated country road, traveling between two villages, I wouldn’t think it was such a big deal.
My Dad tells me stories about my Grandpa letting him take the car to hockey practice at around that age, because Grandpa didn’t want to get out of bed so early, and in those days (1970s) it was seen as good clean fun for a group of kids to buy an old beat up car, take it out on the logging roads, and drive it until it was dead. The kids likely would have been carrying their .22s with them as well, just in case they saw a grouse or a squirrel. The new generation has gotten pretty soft.
The problem obviously arises when the kid crashes into something, or worse, someone. I don’t know how the laws work in Cambodia concerning that kind of situation. I’d think the kid’s family would be screwed if he ran someone over, and the victim’s family would obviously have every right to be furious, and demand a large compensation.
How does it work if an adult with no driver’s license or insurance crashes and kills someone? As others have stated here, the adults don’t drive so well either.

the irony is that if the kid's dad is a driver, presumably a professional, that he taught his son how to drive properly. that fact in itself probably makes the kid more skilled than many adults. I'm projecting this to the thais on the road where I am.