Well I don't know if you would call it a joke...it is considered humor. It is commonly understood that for something to be humorous it must contain an element of truth...even if it is a small element. The truthful element in this exercise demonstrates the fallacy that, as we get older, all of our faculties increase/improve. With age usually comes experience and wisdom (hopefully) but we also get into routines, including the way we think, that can often limit our abilities. These questions and their intended answers show that sometimes an innocent type of common sense, unencumbered by conventional thinking, can figure something out that all the schooling and experience in the world can't. Throw in a cliche underdog reference to how 4 year-olds outperformed the professionals and you get a humorous story that does in fact make a true statement about human nature...even if it is a bit exaggerated.
Now if I can add one of my own:
Does anyone know why elephants paint their toenails red?
