Like antiques, comic books fall into periods known as ages. Like a Victorian Era chair, the age that a book was printed in tells you a lot about the industry and environment of the time. The ages discussed in this node refer directly to North American comic book publishers, as different worldwide comic book publishing industries experienced different defining eras.

The ages are related to comic book grades in that they help determine the relative price based on the period that they represent as well as the age of the book.

Loosely based on an ancient Greek model of history, which described a golden age of mankind before trouble came into the world, the grading of each age towards the modern shows a type of declining scale. This is not to say that comic books where at the pinnacle in the 1940's and have been getting worse since. It is a nostalgic view of history that places the oldest examples of this fragile form of work on a pedestal. It also relates to the increasing scale that collectors are willing to pay for older books. A Good graded Golden Age book will fetch far more at auction than a Mint Silver Age book in most cases.

An important factor in determining the value of a comic book is the age of the book itself. Rarity increases the further back in the ages you go. Any currently surviving books distributed prior to World War II are likely to have survived countless War Era paper drives and many a wet basement and musty attic. This makes them exceptionally rare and highly valued. Production quality of these books also ensured that few survive today. High acid pulp paper and low quality inks make the few surviving examples from the Pre-Golden Age worth considerably more than their weight in gold.

Always a hot topic of debate amongst comic book collectors, the exact start dates of each period are not firmly set in stone. Some widely accepted dates are sited as that start and end of particular periods.

          
1897 - 1937  Platinum or Pre-Golden Age
          
1938 - 1955  Golden Age
          
1956 - 1972  Silver Age
          
1973 - 1985  Bronze Age
          
1986 - now   Modern Age (also called the Plastic, Tin or Iron Age)
          

Here is a summary of the milestone books that mark the beginning of the Ages:

Platinum or Pre-Golden Age
1897-1937
Begins with the first printed collections of weekly comic strips from newspapers. Ends with the Phantom in Ace Comics #11 and Superman in Action Comics #1. Debate over this point is heated.

Golden Age
1938-1955
Begins with Superman in Action Comics #1, June 1938 and Batman in Detective Comics #27, May 1939

Silver Age
1956-1972
Begins with the relaunch of The Flash in Showcase #4, October 1956

Bronze Age
1973-1985
Begins with Gwen Stacy's death in Amazing Spider-Man #121, March 1973 and the relaunch of X-Men in Giant Size X-Men #1, May 1975

Modern Age
1986-current
Begins with Frank Miller's The Dark Knight Returns, March 1986 and the Superman: Man of Steel miniseries, June 1986

It should be noted that this model changes as the Modern Age continually creeps into the future. The relative ranges of the older ages slowly increase over the years. The division between the Bronze and Modern Ages is particularly liquid, as the defining milestones of each age are widely debated.

Another point to consider is the fact that the setting of a story may be described as belonging to a certain age. A Modern day series may be set in the Golden Age, and the storylines and heroes would be stereotypical of the type of book that you would associate with that period of time. The milestone books generally note the first real example of the type of book that would dominate the age. These transitions note interesting changes in direction for the comic book industry.

Pre-Golden to Golden Age
The comic strip collections from daily papers gave way to the Superhero with the dawn of the Golden Age. With the Phantom and Superman, writers for the first time wrote of men who were more than adventurers. They had strange and fantastic powers. The contemporary villains of the early Golden Age reflected the modern culture of the late 1930's radio drama. Bank robbers and gun-toting gangsters abounded, and the fantastic properties of chemistry and radium where common to storylines. Villains were vicious and homicidal, more so than in some of today's adult focused books. The Batman of the 1940s was a grim killer.

Golden to Silver Age
The Cold War and the rebirth of the superhero genre both began at the end of World War II. Stories and settings of the Silver Age are decidedly more surreal and fantastic that those of the Golden Age, and the heroes more the textbook Superhero type. The softer subject matter and wild fantasy also give insight into the industry. The Comics Code Authority and the constant threat of the Cold War made it easier to base your stories in depths of space, or alternate universes than in Chicago or Moscow. Villains of the time where faceless secret societies, interstellar threats and hideous monsters born of radiation and the new atomic science. Stan Lee's characters and Jack Kirby's art redefined the superhero in the early 1960's with the likes of the Fantastic Four, a family accidentally made heroes and Spiderman, a nerdy boy who had the problems of both a superhero and a teenager. Giving the hero a human side signaled the end of the Silver Age.

Silver Age to Bronze Age
In giving their heroes a human side, as well as introducing the Bullpen concept of creative management, Kirby and Lee spawned the Bronze Age. The human side of heroes came to dominate the stories of the Bronze Age. Heroes had to deal with death, drugs, the conseqences of their actions. Suffering, drama, fear, love and loss all became common to stories. Conflict between heroes and the prejudice of mankind as show in the revived X-men showed more and more of the reality of life. The gritty details were being shown for the first time.

Bronze Age to Modern Age
The most debated transitional period is that of the change from the Bronze to Modern Age. The rise of the anti-hero is landmark event that created the split. Frank Miller's work on Batman showed dark and driven Bruce Wayne, a man fighting to control himself as much as the criminals he faced. The Punisher, a man driven to avenge his families deaths by killing, is another example. The surreal and gritty anti-heroes are a thoroughly modern fixture. The addition of adult themes and stories and the idea that the hero can be just as flawed as the villain are new to the Modern age.

Sources -
Wizard magazine - Jan 2000
ebay collectables listings
Pseudo_Intellectual's massive brain