User blog comment:Thailog/Ep. 206 "Bloodlines" discussion/@comment-4962084-20120602010540/@comment-1839689-20120602015858

I think some of it is his stance and the framing, too. In that ta-da moment, he's stretched to his full height and then some. Face-to-face with Gar, Bart's slouching, making his eye-level a bit below Gar's. Gar's stance isn't straight either. It looks like, if they were both standing straight, they would be about the same height. Gar then acts as a reference point. Since we know Gar is thirteen, and one of the shortest members of the team because of it, we realize Bart's age most be pretty close to that.