Well, Amano says that Xanxus is twenty-four when he shows up for the Ring Battles... the timeline is a little fuzzy, but the Cradle took place eight years before that.

I have decided, for my purposes, that what happened is Xanxus got put on ice as a teenager (sixteen) and that he has not actually aged at all while frozen. So, chronologically, he's technically twenty-four; in lived experience (and to an extent, mentally), he's still sixteen. Which is a hell of a lot closer to Tsunako's fourteen, is all I'm saying.