What's the typical age that kids begin drawing with perspective? For example, when do they normally stop drawing pictures with a stripe of blue at the top for the sky and a stripe of green at the bottom for grass?

I'm asking because today, DS4.4 drew a picture of a house and began coloring green for grass around it. Then he said, "I'm coloring green on top of the house. That makes it look like the grass is behind the house."

Frankly, I'm hopeless at art, and if I were drawing the house, I'd probably stick with the strip of green at the bottom and blue at the top and call it good. So I'm probably easily impressed. :-) But since I'm already thinking about putting together a portfolio for DYS, would this be something worth documenting and including?