Is he really just 4?

Benjamin amazed me today. Each morning when we get to school, Benjamin loves to write or draw on my whiteboard. He will usually write the latest letter or number he has learned. Today he said he was going to write the number 10. I said okay absently as I was trying to get organized for the day. He did and it was a great 10. The next thing I know he's saying, "There is another way to write a 10." I said, "Okay, do it." I looked away and while I was busy he wrote 2 5's. Then he said, "5 + 5 is 10." I whipped my head around and realized that Benjamin had written his first number sentence!!!! He is really just 4?