Okay so Mike was really nice and he even drew me a chibi Aang in my avatar artbook after signing it! As for the questions, I only got to really ask him 2 questions cause I didn’t want to be rude to others in line.
The reason why Aang appears to be in his younger body when he’s a spirit is apparently because they wanted him to appear in his most iconic look. Bryke wanted each avatar to appear as they did in their most iconic scenes. Mike also mentioned that this is also a reflection of how the Avatars want to be presented when seeing new avatars, so that’s the reasoning behind Aang appearing as a young guy and Roku appearing as his older self.
As for the glowing eyes thing he seemed to be a bit confused by the question. I think I may have phrased it oddly. He basically just said that Korra hasn’t fully mastered the avatar state yet, so that’s why the eye glowing thing is different. Sorry Stephanie!