Symptoms definitely can and do change as LOs mature. Take, for instance, the DS of one of my dear friends IRL (the boy will be turning 7 in July). As a baby he would projectile vomit from dairy. By a year he was no longer vomiting so she thought it was OK but he started getting near constant ear infections, then after tubes it turned to asthma, etc. Now he's off dairy (and eggs) again and the congestion is almost gone!