evidence: Why it's dangerous for newborn babies to drink water [...] The average, healthy adult is made up of 55-60% water. The average baby, on the other hand, is roughly 75% water. And that difference is why babies shouldn't drink water before they're at least 6 months old. Not from the tap, or a well, or a spring. Plain water is just plain off limits. [...] But it's not just drinking straight H2O that poses a threat. In fact, most cases of water intoxication in infants don't even involve a glass of water. A common mistake is when people dilute baby formula TOO MUCH, by accident, or when parents dunk their infants up and down in a swimming pool, and in the process, the infant gulps in too much water.