Well... The son I've got in mind (the one that's showing an interest in this subject) is 16. Age isn't a factor.
My plan is to use Python as a stepping stone: teach some basic procedural things, use turtle to provide some visuals (he's taking Drafting classes at his high school), and then introduce him to the tools provided with the Unreal engine (A friend of mine pointed me at this video as well. It should be pretty interesting.)
I'm not planning to leave him high-and-dry; I'm expecting to be learning some stuff as we go, too. He keeps talking about wanting to design his own games, so I wanted to be able to steer him toward some tools that would help him reach his goal.