Do you have a specific image or set of images in mind? There are quite a few cases where babies can look like babies, small children, or small adults. Sometimes, for example, you'll notice that everyone has basically the same face. It's helpful to me to think of those as essentially cartoons (think of xkcd today! We have no trouble parsing those characters as different based on clothes or a couple of scribbles to signify a beard).

In other times, the "baby" is not a baby at all, but rather signifies the soul of the person holding it/next to it. (Or in pictures of Judgment, sometimes an angel guides what looks like a small child--again, a soul, and likely of someone who had died as an adult).

Without a specific illumination or painting, though, it's difficult to tell in the individual case! ;)