Well, it is true people say that. Been saying that for as long as I can remember. Really makes no sense but gives the sayer something to say, is what I figure. Don't let it bother you. There is much more out there that will bother you for real.
About Adam and Eve? I suppose they both had belly buttons, although I cannot prove it.
What that refers to, is stay a kid in your heart, not actually be a big kid, and act like a child, that's why people tell you to grow up. As for the belly button thing, I don't know.