Go With The Flow
Since at least the 17th century, people have been saying that a dubious idea or explanation “doesn’t hold water.” The allusion is to a useless container which can’t carry liquid, the same idea conveyed when calling a story “full of holes.”
Newborn babies are typically wet with amniotic fluid, so “wet behind the ears” means someone new and inexperienced. Curiously, the term “dry behind the years,” for an experienced person, seems to have been first used around the same time, but has not survived into common parlance.
A momentous life happening is sometimes called a “watershed moment.” This is a geological reference, since a watershed can be a ridge or mountain chain (like the Great Divide) that defines the direction which water flows down either side of it.