I wish I knew, and actually some keep 91 as part of the Bronze Age (myself included) and that would make 1992 the start of the Modern Age. Now if that is the case, I could make the argument easily that Image Comics starting up is what flagged the Modern Age versus Copper.

If I had my druthers, everything would be done by decade and date alone, it is weird when the Ages are based on art styles and story types or things of that nature.