These are just rough estimates, as when you get down to specific titles, not everything fits neatly between the cutoff lines in a completely logical way, and obviously the broad eras fit the superhero genre (as the most collected) better than anything else -- particularly with reference to the cutoff point between the Golden and Silver ages. Also, these tend to become fairly meaningless terms when you're not talking specifically about AMERICAN comic books. Then again, these eras can be further subdivided. It's just as relevant (if not more so) to refer to 1940s comics, 1950s comics, 1960s comics, etc. One thing about using various metals to describe an "Age" is that you would expect Platinum to be more valuable than Gold, yet "Platinum Age" comics are often, or on average, worth less in monetary terms than Golden Age comics, so the term is counter-intuitive. "Modern Age" barely seems like a reasonable term to encompass ALL of the American comic books published for the last 30-ish years, as if there's no functional difference between a comic from 1987 and one from 2017. You often encounter different terminology, such as "post-war" or "pre-Code" or "Good Girl Art" which are useful in describing things for more specific reasons. At any rate, I never felt like these unofficial designations were anything to get too worked up over.
Also, with reference to the original designations of Golden-Silver-Bronze as defining Ages of comics, these are based on the classical Greek "Ages of Man" as defined by the poet Hesiod, so if we continued following that model, the next ages would be the Heroic Age, followed by the Iron Age. The terms Bronze Age and Iron Age were actually adopted by historians to describe the evolution of civilization, in terms of metal-working technology. That progression follows in the order: Ice Age, Stone Age, Bronze Age, Iron Age. Some have added "Industrial Age" (or "Machine Age") and "Atomic Age" and even "Digital Age" (or "Computer Age") to the list, similarly based on development of technology. In between you have all sorts of various ages, based on various developments of Western Culture: The Age of Rome, the Middle (or Dark) Ages, the Renaissance Age (or Age of Enlightenment), and even the Steam Age. Not to mention the Elizabethan Age, the Victorian Age, the Edwardian Age, etc. Obviously, depending on the perspective of who's using whatever terminology, you can't get everyone to agree on what "Age" any particular year might belong to, so it just goes to show that nothing's set in stone.
https://en.wikipedia.org/wiki/Ages_of_Man