Colloquially used to refer to comics from 1970-1979. The acknowledged catalysts of the Bronze-Age are the first appearance of the Punisher
(Amazing Spider-Man #129
), the Death of Gwen Stacy
(Amazing Spider-Man #121
), and Green Lantern #76
of Green Lantern
's storyline where Green Arrow
was added as a title character.
The introduction of the "Copper Age" has pushed back the boundary of the Bronze-Age to include years through 1982 generally, and possibly 1983 as well. (Keep in mind comic era boundaries are fuzzy, and some comics may continue to read like the older ones, and thus belong to the older era).
See the Overstreet Price Guide 2004 for their discussion on eras.