Capes - you hardly see them any more! Were they cool? Try dissing Dracula; or Dr. Strange!
Handkerchiefs. I used them when deployed to the Middle East or Afghanistan. Used as a mask to keep the dust out of my lungs - a bit. Otherwise used as sweatband. Made a comeback when COVID hit as a temporary better-than-nothing mask.
And risk getting sucked into a jet engine? No thanks!
When I was in high school, the only acceptable way to wear a backpack was on a single shoulder. Wearing it with arms through both loops was Prime Dweeb Behavior. Watching the students filling out today, I noticed they all wore their backpacks on both shoulders. I can’t say that they wore them properly since they were all low slung and probably not doing their spines any favors or support but they were closer than we got. I suppose wearing my backpack c.1989 style would brand me as a weirdo.
Cargo pants.
Well, then, I have no claim to glory, driving a Prius, which was never cool. Though, I’ll grant, the last couple of model years have much cooler styling and substantial additional power.
Speaking of cars though - the Volkswagon Beetle went from fugly but effective, to counter-culture cool (and pop culture if you like the Herbie movies), to retro-cool, to anti-cool with the New Models, to possibly retro-cool again, or possibly mostly forgotten.
Another example for this thread: calling uncool people “dweebs.” 
Doesn’t work on the handcycle. I don’t have a car. The first aid kits alone wouldn’t fit in cargo pockets.
Funnily enough, I saw a new Broadway musical comedy last night that had a scene where a 20-something dorky young man offers a woman his handkerchief, in another example of how cringingly uncool he is, and she recoils in horror seeing that it was previously used.
I couldn’t help but think of this thread!
For all those folks that are aghast at cotton fiber being used to absorb bodily emissions, what do you wear around your pelvic region? Kleenex brand undergarments? Adult Pampers?
You do know you are packing around garments with bodily emissions that you throw in the laundry? Are the pelvic area emissions so much better than those out the nasal passages?
OMG, you blow your nose in that and then put it in your pocket next to your soiled underwear?!?!
Just how much is on your underwear that it compares to what comes from blowing your nose? 
A pocketful full of snot is gross to carry around and keep using.
This is crazy talk; I see plenty of young people wearing jeans.
But if you mean things like the style, fit, and cut of the jeans, then I agree those things can sometimes be age signals.
When you have used it, you don’t keep using it. You wash it.
You should have been at the Hollywood Roosevelt Hotel the weekend Jethro Tull played the Greek and James Taylor played the Bowl. It illustrated this point perfectly.
We were in town for the Tull concert.
That tearing sound!
I see I’m days behind you with the Seinfeld references.
No, just Generation X.
Or early Millennial.
You mean “old geezer” 
Where did the term geezer come from?
One person’s opinion:
One of the slang words used in the American theatrical profession is ‘geezer,’ and it is applied to one who is garrulous or loquacious. I fancied that the word must have originated in the contempt in which the people of Guise were held by their neighbors; again I suspected that the opprobrium might have originated in ‘goose’ or ‘geese.’ But it finally occurs to me that it is a corruption of the word ‘geyser,’ which means ‘a gusher.’
–Eugene Field, London, from “The Chicago Daily News” Feb. 19, 1890
So originally it didn’t mean ‘old guy/gal,’ but I guess what we might call today a ‘motormouth,’ and contemporary references seem to back that up. In the late 1890s there was a play called “The Geezer,” which appears to be a parody of then-current play “The Geisha.” But by the turn of the century, the term ‘old geezer’ started gaining in currency, and the connotations of verbosity were ultimately supplanted by connotations of age.
Side note: In the our gang short “Joy Scouts” (1938), Alfalfa reads the sign “Geyser Springs” as “Geezer.”