Even discounting that this is Chicago, and therefore the kid probably got held back, it is possible:
Kid turns 6 just after the cutoff to start kindergarten, so has to wait one year, and thus is 6-7 in K, 7-8 in 1st, 8-9 in 2nd, 9-10 in 3rd, 10-11 in 4th, 11-12 in 5th, so 12-13 in 6th.
When I was in grade school back in the 80s, there was a 14-yo in 6th grade with me. His name was Samnang Sam, and he was a Cambodian refugee who had been brought to the US having never been to school before. I ended up playing left winger on the same junior soccer league team with him. They had him in at center, and between me and him, we won every single game. He’d steal the ball from the other team’s center, tip it to me, I’d run it down the sideline, kick it back in to him, and he’d score.
Nice memories for you! I wonder what Sam's doing today.