There’s two sorts of “stuff” anyone owns. Practical stuff and nostalgia stuff.
For the OP’s teens, the sheer amount of practical stuff a teen “needs” today is far less than in my teen era 50 years ago.
E.g. all teens of all eras want music. That meant in my youthful era having at least a 2 or 3-component stereo, plus two external speakers, plus physical artifacts holding music to play, be those LPs, 8-tracks or cassettes. And cleaning tools and wires and maybe wiring tools or an assortment of patch cables. Nowadays it means owning a phone and earbuds. Which serve many purposes beyond just music playback. Less stuff.
Back then most kids with enough money had cars, and the guy-kids had tools to clean or fiddle with their cars. Which cars actually needed fiddling with. Now cars are reliable sealed appliances and a hell of a lot of teens have a scooter which replaces a bike, and a car, and maybe a motorcycle. Less stuff.
A modern kid of the same SES I grew up in probably has owned 3 or 5x the items I did by age 15 versus them by age 15. But they’re almost all treated as disposable, not to be saved. Whereas very little of my childhood crap had been gotten rid of by the time I was 15. It still worked, so it was saved. Al of which leads to a much more “if I’m not actively using it, just get rid of it” mentality in everyone.
This all seems real simple and obvious to me.
Now where it gets fuzzy to me is the nostalgia stuff, like old softball uniforms, trophies, etc. What follows is rampant speculation: Kids nowadays are much more likely to be involved in organized activities than in my youth. Yes, Little League has been around since my long dead father was a kid.
Much of the nostalgia-ware comes from organized activities: trophies, ribbons, plaques, team pictures, uniforms, etc. I played organized school sports 3 years in high school, so just 1 year by age 15. A modern 15 kid will be enrolled in two to four organized activities (sports, dance, extra curricular art, music lessons, etc) every year from age 4 to age 15. They’ll have a lot less interest in saving an e.g. team picture or band t-shirt because they already have had lots of them. They’ve been throwing away the old ones to make room for the new since they can remember. And they’re confident they’ll have new ones next year to replace what they’re selling now.
As to the OP’s specific example of that specific estate sale, the fact the sale is selling dirty laundry in a hamper makes it look like the teens were removed from the home rather urgently. The OP was right to check to see if they’d died in whatever tragedy took the oldster(s).
But other possibilities exist: They were staying with the grandparents because they have no other functioning adults in their extended family and they are now residing with Child Protective Services. Or they were just here for the summer and do have real parents, but those parents live a thousand miles away and as soon as the grandparents died, the kids were unceremoniously whisked back to home elsewhere leaving a mess of dirty laundry and random possessions they had neither time nor space to pack into a suitcase for a flight home.