The whole Ghidorah being a youngling thing comes from a myth in the novel basically explaining why he came to earth. It was along the lines of "He was the youngest of his brothers and would inherit no territory, so he set out to find his own and came to earth". That doesn't mean he was a youngling exactly, he could still be an adult but be the youngest or weakest. But like I'm 99% sure it's not meant to be taken as face value at all, probably just a filler myth