Cambodia is not known for its laws and their enforcement. The country just hasn’t advanced to the stage where that is important, especially when everything depends on the whim of a strongman (aka dictator). I have asked several people what is the minimum age for driving. None of them knew. Maybe there is a legal minimum, maybe not. Either way, it’s not important. The prevailing sentiment seems to be that if your feet reach the pedals, you’re good to go.
Here are some photos of Phnom Penh’s less experienced drivers