12 year old can’t fight in fictional worlds?
Without superpowers? No. You won’t see me saying Billy Batson is unrealistic because he’s 12. But, if you want me to tell you how your unpowered 12 year old can fight adults, the answer is, “they can’t.” Full stop.
And, yes, Avatar is a series that is all about superpowers. If you honestly believe any practitioner of Hung Gar or Southern Praying Mantis can do that, I’m afraid I have some sad news for you.
If you want to watch kids practicing martial arts, you are much better off checking Wuxia films. Keep in mind that most of the genre isn’t known for it’s realism, but you’ll get a better sense for what kids can realistically get away with. Were you actually making an argument this clip from The Iron Monkey would be a better choice. However, fantastic martial arts all around. Unrealistic in the true Wuxia style, but fantastic. I’ll also leave this clip from the same movie for comparison. Try not to look for the wires.