I can't tell huge differences between the 1920s, 1930s and 1940s, or the 1990s, 2000s and 2010s, but it's easy to tell if something is from 1946-1962, 1963-1969, 1970-1979 or 1980-1990.
What's distinct about the Cold War period that made pop culture and style bloc into readily identifiable roughly 10-year periods? My pet theory is the prevalence of visual one-way electronic communication, more specifically television creating a monoculture in developed parts of the world. Before then it seems like it was harder to create a strong and uniform style that's readily recognizable, and trends changed more slowly. After then you had the Internet, cable and a more splintered music scene, meaning that there wasn't as much common culture between people and making it difficult to tell post-1990 decades apart from each other.
Your pet theory is interesting, but what's actually happening is that you're only aware of the stereotypes of the Cold War period - the earlier ones are perhaps outside of your general sphere of interest, and if you aren't used to looking critically at the aspects of material culture that make up these stereotypes (clothing, hairstyles, interior design, car design, trendy colors, etc. etc.), you may not notice them on their own in more recent years. I'll assume that you're roughly my age or a bit younger - a Millennial, that is - in which case, you've experienced a lot of 20th century period pieces created by Baby Boomers that explore the era of their youth, celebrating and/or exaggerating those aspects of material culture mentioned above, and making them more obvious to you, ranging from the scrupulous reproduction and serious drama of Mad Men to a sitcom like That '70s Show. And of course, we have a lot of media from that time available today! While pre-WWII films have generally been a bit more obscure for various reasons, a huge number of classic and enduringly popular movies date to the 1940s and later.
I'd also quote myself from a previous answer on changes in decorative arts in the 19th century:
A lot of the characteristics assigned to specific decades of the twentieth century aren't actually specific to them at all - the end of each decade strongly resembles the next, and the beginning of each decade strongly resembles the last, because when looking at decorative arts, fashion, music, etc. defining a decade as 19X0-19X9 is fairly arbitrary.
Which is to say, while the popular perceptions of these decades is that they're strongly defined, they're just as much of a smooth progression of changes as 2000-present is, rather than being clearly-defined boxes of "types".
I'm going to focus just for now on the 1920s-1940s because you mention them specifically, and because that's a doable bite of history. Sticking to the concept of a decade-defining image, let's start with women's daywear:
Pattern illustration, May 1940
Do you see that the 1920s women have no waist apparent in their clothes, brimless hats pulled down low, and and overall loose but tubular silhouette? Meanwhile, the 1930s women have close-fitting dresses with a slightly high and belted waist and a little flare to the bottom of the long skirt, and brimmed hats that sit cocked to one side of the head, and the 1940 women have puffy sleeves, a strongly defined waistline, a noticeable flare to the shorter skirt, and quite a brimmed hat.
Or men:
The differences here are more subtle, but still apparent. The men from 1922 have defined shoulders with a smooth, curving line down to the hem of their coats, starched standing collars (and one has a bow-tie), cuffed pants, bowler hats, and small Hitleresque moustaches; the 1932 man's coat is shorter, his pants cuffless, his collar turn-down, his shoulders sharper, and his moustache thin; the 1942 men have even broader shoulders and lapels, fuller (cuffed) pants, and fedora hats.
Let's try cars, which I do not have the vocabulary for:
Photo of 1933 Auburn cabriolet
Car from National Convention, 1940
The 1920 illustration of a car shows it as pretty spare and angular, with thin fenders over each tire, a squared-off front, and uncovered headlights. The 1933 Auburn is similar, but the fenders are a bit thicker/curvier and flared and the back is curved instead of being a straight slope down.The 1940 convention car has fenders that meet the body more smoothly, the headlights are covered to match, the hood curves down to the front instead of being chopped squarely off, and the roof slides down to the rear bumper instead of having a really defined trunk that sticks out in the back. My point is, visually-apparent changes occurred in the early 20th century happened just as they did later, and if you're as used to them as to those of 1945-1989, you can see them easily.
I don't want to really get into the post-1990 side of the equation due to the 20 year rule, but if you watch Clueless (1995), A Cinderella Story (2004), and a few episodes of Younger (2015-), all back-to-back, I think you would notice some really distinctive 1990s, 2000s, and 2010s styles in the above categories.
My pet theory is the prevalence of visual one-way electronic communication, more specifically television creating a monoculture in developed parts of the world. Before then it seems like it was harder to create a strong and uniform style that's readily recognizable, and trends changed more slowly.
Television was not very revolutionary in disseminating styles of dress and design, perhaps because the monthly magazine format that had been doing so since the late 18th century was sufficient - a month-to-month basis is even a little excessive for these things, since nobody buys a new car, revamps their wardrobe, renovates their house, etc. every month, so the theoretically moment-to-moment changes a television could bring were completely unnecessary. If anything, your period of interest is actually the time of the rise of stronger and more widespread countercultures (e.g. the back-to-the-earth hippies, punks, goths) with street styles that were not based on what fashion designers and retailers wanted to spread via mass media.