Delayed gratification at pre-school age is associated with higher SAT scores, interpersonal skills, and a lot more good things. A mom in my class tried to give this test to her son, and he just said, "Well, fine. I'll just take this one now." So we don't think it works if it's someone you're not afraid of offending.
Notice the distraction technique for the boy who makes it the whole way through - he looks everywhere else for a while!
P.S. Sitting in Pittsburgh right now. Working on a paper. (See how well that's going?)