Talking with our pediatrician this week for the kids' checkups.
She says that children have trouble drawing one line across another until about age three and a-half. They just can't bring themselves to do it. The doctor asked Sophie (three years old) to draw a + and sure enough, she had trouble doing it. She kept drawing vertical lines, one after another until the doctor showed her again what she wanted her to try. Even then her horizontal line was labored and misplaced, and Sophie seemed a bit confused as to the whole thing.
Tony Millionaire has said that he doesn't like to draw one line through another either.