For potential athletes, many countries begin around twelve. A lot of Pro boxers started at 9 or 10, as some gymnast do.
For general weight training twelve can be acceptable for most. With your step kid, might want to start him off learning the basic Olympic lifting movements; cleans, squat cleans, snatches, jerks, etc. Throw in military presses and benches if you wish. All with light and higher reps. Nothing heavy, because of too much joint pressures. If he becomes interested in sports the Olympic lifts will greatly improve his timing, balance, quicknes, stretch and flexibility. Plus all around power. The secret is to make it enjoyable and fun for him.
If he's like most young boys, he wants muscles. Usually starting with the biceps and pec's. (some guy's in their 40's, working out most of their lives, never out grow the need for over doing the arm & pec work. Avoiding the legs and back for the most part).. If he's like that than start with a all around BB'ing workout. Squats, benches, rows, presses, curls, etc. Make it light and with higher reps at first. Three workouts a week with all the basic stuff. Good Luck.