All things being neutral, I'd probably have a *big* propblem with my son gaming with a much older person as a minor. Unless I knew the person like a brother/sister, I'd have to wonder why they wanted to play with a kid. no matter what they said, it would still be in the back of my mind, and I'd dissallow my son from taking part without me or another adult I seriously trusted. I don't see any other way to be a good parent in this situation.
The trouble is, there really are bad people out there (maybe not in the numbers many would have us believe), and the ones who have a thing for kids go where the kids are . I'd have no malice for the person who invited my son to play, but I wouldn't invite the possibility of trouble. Strangers are strangers, even if they share the same hobby as me.