I was thinking about how we're approaching the year 2020, and as such the term "the twenties" will become more ambiguous, since it could then refer to either the 1920's or the 2020's. Is this something that occurs every century? For example, did the term "the sixties" refer to the 1860's up until 1960 rolled around?
Follow up question that applies mostly to British culture. When did we begin to describe periods as the '20s' instead of as 'Georgian' for example, and why did this change happen?
Not sure how I'd go about compiling statistical data on usage rates or anything, but I've definitely seen that terminology used in 19th century American sources.
Here's one published in 1903 which discusses the 1800s. Search for "sixties," "fifties," etc and see for yourself. They used terms like that fairly often and I have no reason to think this source is somehow special.
One example I found recently: Korniliy Aleksandrovich Borozdin was the Russian governor of Telavi region (part of Georgia) in 1861-1865. In his memoirs he described his service and problems he faced in the recently pacified region, trying to achieve compromise between local Georgians and Lezgins. In the chapter "Alazani Valley", he writes: "In the early fourties, when we had a post in this village, a Lezgin raiding party massacred our entire staff, including a young clerk, one Sukyasov." Fourties here meaning 1840s, and the memoirs were written in 1886.
(source in Russian: http://www.vostlit.info/Texts/Dokumenty/Kavkaz/XIX/1860-1880/Borozdin_K_A/text1.htm )