Adage v. Bromide
I suppose this isn't as complicated as I originally thought. Adage is an old saying which has taken on common usage by virtue of the amount of its usage, where a bromide is a dull or boring saying which is just common, inherently to the saying and not the usage. Adage comes from the Latin for "I say" (go figure), and bromide has a rather circuitous route from the element, bromine, which was apparently used as a sedative, who would make such dull/boring sayings. That's it. So, personally, I think bromide is itself a dull word, plus it would always sound like its elemental origins. "Just wait until you have children" is such an overused bromide. Certainly, there has to be a better parental retort than that??? "Look both ways before you cross the street" was never really a bromide, but more good advice and better still, a defense against liability. The warning "don't touch" is a pointless bromide for the stupid? I'll just stick with adage. I will note, however, that saying "old adage" is redundant, since adages are already inherently old. That said... "You are what you eat" and "think before you speak" are familiar adages, while "Plaintiffs lie" and "think before you litigate" should be.