The primary issues are with cow's milk for young infants, or cow's milk becoming a large part of the diet, at too young an age.
1. Cow's milk contains inadequate vitamin C and iron, and too much protein relative to fat. If a baby or young toddler drinks it, their appetites for more appropriate foods are diminished, and there's a risk that they will not get enough of those nutrients that they need.
2. Too much protein puts stress on the kidneys.
3. The protein casein is difficult to digest and allergenic.
That said, my kids all ate cheese and cow's milk yogurt at that age, so I can't see that occasional sips of cow's milk are a problem as long as it stays with just small amounts. Some people delay potentially allergenic foods a long time, but I'm not one of those.
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