I don't know if the 1920s were necessarily the start of the "modern" era, because as someone else mentioned, "modern" is relative.
If anything, the 1920s was when the "real flavor" of the 20th Century started, and by that, I mean the look of it, and technology. Phones, radios and cars were all commonplace by the 1920s. Social mores were also much different than in the previous generation. IMO, the 1920s were kind of like the 1960s, in that both decades are associated with a social and sexual revolution, in regards to how western women dressed and behaved, and there was that feeling that things would never return to how they were. Which is funny, because you can interpret the 1950s as being a step backwards in terms of a "woman's place," and it's even reflected in the fashions of the 1950s.
Here's a clip of Joan Crawford in a "talkie" from 1929. Her outfit was pretty much the standard by then. Can you imagine a woman from 1909, just 20 years earlier, being able to show her legs in public, and show her bare arms outside of evening wear? And dancing around like that? Scandalous!
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