In news and opinion that should surprise absolutely nobody who has a lick of common sense, scientists now understand that giving young children (babies really, pre-weaning) antibiotics is what causes the massive surge in asthma, allergies, and closely related health problems that is sweeping the developed world. To wit:
If antibiotics were given when the mice were weaned and no longer in the care of their mothers, there was no effect in susceptibility to asthma. There appeared to be a critical window of time, early in life, during which antibiotics had an effect on the development of asthma. When given orally, the antibiotic that we chose, vancomycin, kills only intestinal bacteria, and does not get absorbed into the blood, lungs, or other organs. This finding implied that the antibiotic-driven change in the intestinal bacteria caused the increase in the severity of asthma, a disease of the lungs! This experiment, as well as others from several different labs, came to the same conclusion: modifying the microbes that live within us at the beginning of our life can have drastic and detrimental health effects later in life.
Bluntly, unless you wish your child to be sick (or unless they have severe post-natal illness), do not give them medication until past the weaning stage.