Follow the motion picture rating system. If your games would be rated R (language, sexual situations, depictions of extreme violence, drug abuse, etc. ... think Deadpool, John Wick, or Tarantino films), then it's probably not okay to include a minor unless one of his parents also participates AND you have a frank discussion about tone and content ahead of time.
On the other hand, if your games would be PG-13 (rare F-bombs, mostly "tame" violence, nothing too grim or gritty ... like the Ocean's 11 or Fast & Furious franchises), then it's probably okay for younger players. To be on the safe side, I'd still insist on at least a phone conversation with one of the kids' parents to address any concerns they might have about their offspring hanging out with people twice his age.
When I run games at conventions, I try to keep the atmosphere equivalent to episodes of "Leverage" or "The A-Team." I occasionally have younger players (as young as 13 accompanied by a parent). When kids are present, I remind the older players to "keep it clean," and no one's ever had any trouble with that.