New research following more than 4,000 children has shown that a high junk food diet before the age of 3 led to a significantly lower IQ later in life, regardless of later diets. Conversely, a healthy diet as an infant gave an IQ increase, but after that it didn't make a difference. Of course, the study only tracked the children until the age of 8, so that's not a license to start gorging on chocolate-covered potato chips after eating only salad until the age of 18.