Interesting thread.
In my my experience, Americans generally refer to things by the year.
"That house is from the 70's"
"That dress is from the 30's"
Or by the decade, rather. Things tend to drift and change roughly every 10 years since the turn of the 20th century, and we tend to define styles and the cultural zeitgeist by them.
The eras are generally defined as such...
18th century: Founding of the country, generally referred to as the Colonial or Federal era.
19th century: The industrial revolution, the Civil War (which is always capitalized, because it was our civil war), and expansion into the Wild West. Victorian usually refers more to style and architecture of the era rather than the era itself.
The 20th century starts getting more specific.
When you see flapper girls, speakeasies, hear mention of The Prohibition, and the resulting gangster wars, you immediately think of the 20's.
The dust bowl, soup lines, and all around miserable people looking miserable in every picture taken? The 30's.
WWII made enough of an impact for it to be referred to directly, so you rarely ever hear anything looking like the 40's. It's always the WWII era and postwar era.
The 50's? Rock 'n Roll, all the trappings of the Lost Generation (I think, need to check on that), generic brightly colored but otherwise generic houses, neon signs, and slickers. The Cold War started kicking in around then.
The 60's? Hippies, Woodstock, the Summer of Love, all around political strife, the moon, and the Equal Rights movement. The Cold War was in full force by this point.
The 70's? EVERYTHING WAS MADE OF WOOD! Also punk music and Kiss towards the end of the decade. And The Cold War.
The 80's? Cocaine, new wave, hair metal, big, big, BIG hair (which I think was why hair metal was so popular), corporate power and the love of money, fear of the Japanese taking over the country. The Cold War got really scary, then ended with a cheer rather than a mushroom cloud.
The 90's had a transitional phase. The earlier part of the decade was all about bright colors, still big hair, and cheese...
(Pure era example right here. Laugh all you want. You know we all looked like this between '89-'92", also Australian style, circa 2006)
...with the later half being overtaken by grunge, a mainstream explosion of "the underground", a brief but bright English alternative music scene invasion, and the widespread birth of the internet. It also marks the last time the country was generally in what could be considered a decent state.
The 21st century? Everything after September 11th, 2001. Everyone is bitchy, whiny, polarized, stubborn, and unemployed. I think the current era hasn't seen any huge transformation since we've segued into the first decade of the century.
And that's a good rough outline of The American Eras.
I expect to get an A- on this at least.
