From what I understand, different ages of people tend to have different sorts of immune responses. While vaccines are safe and effective, it takes some time to get the dosages right for children because of the way their immune system responds to things - at least, with a new vaccine. I'm not an expert, though, and I got this information while looking up variances in side effects (older folks get less than younger folks, and folks with two x chromosomes - mostly females - tend to get more as well).