No no no, it can all be explained away with lore. The child is the child of Mr Fantastic and the Invisible Woman! He inherited both their powers but they occur in strange ways. First, his body does not stretch like you know of Mr Fantastic, instead he is able to simply make certain parts of his body much larger than they normally ought to be. While this happens, the rest of his body parts shrink, often giving him the appearance of a baby, but if you could see his face, it'd be that of an adult man. And that leads me to his SECOND power. He inherited invisibility. But again not how you think, instead he is able to hide his age at will! He's actually a 20 year old man, but he can look like a baby if he wants to!
So that should explain all that. Oh, and he's unable to hide his powers when his mother breastfeeds him, and she still breastfeeds him in his twenties because they don't believe in that habit being aged out. Momma's milk is nutritious for a growing young man.