Why babies can't drink water by Business Insider While adults need a constant reminder to drink more and stay hydrated, it's a different story for newborn babies. Their bodies aren't developed enough to consume even a few ounces of water, which in extreme cases could be fatal. Following is a transcript of the video. There is a time in every person's life when drinking water is deadly. Even as much as a few ounces is enough to kill. 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. You see, everyone's kidneys have a limit to how much water they can handle. Break that limit, and the excess water will back up into your bloodstream, where it dilutes the salt, or sodium, in your blood. Once your blood dips below ...