I would advise you not to worry.
1) Any friend worth his salt, online or in real life, will accept you for who you are, and not hold anything against you as long as you haven't been intentionally deceptive, which I have no doubts that you haven't. And if anyone is so pretentious as to get offended that you aren't Japanese, or aren't an adult, you probably didn't want to associate with those kinds of people anyway. So no loss there.
2) People at the con are generally very forgiving and accepting, especially the people I've seen here. For one, the con is one of the few places where you're allowed to completely "geek out", because everyone else there is there for the same thing. I would not worry one bit about it. Worst case scenario, someone get's mildly suprised, not offended.