Here are my general questions on this subject:
(1) Do most older people really get ticked off when a service employee says “no problem” instead of “you’re welcome”? Judging from social media that certainly seems to be the case.
(2) Does something parallel to this exist in other languages or cultures?
And here is a question I’d like people to answer regarding their own feelings on the matter, only if they wish to, of course.
(3) Does it bother you if a barista or whoever says “no problem”?
No problem. Not at all. No worries.
But call me by my first name when you’re younger than me, performing a service for me, and we’ve never met before, … well, that’s a problem. Respect is earned.
There’s your problem.
Respect is earned? But you’re entitled to it by virtue of your age? I don’t follow, Greg.
It used to grate on me. I’m expressing gratitude, not apologizing for being a bother. Just accept my thanks!
However, someone pointed out to me that “de nada” in Spanish is essentially the same thing, as is “de rien” in French. And even old people have been saying things like “don’t mention it” or “it’s nothing” in response to thanks. It’s just the new construction that caught my ear and grated, so I decided to get used to it.
I am “mostly older” but “no problem” makes more sense to me than “your welcome” in this context.
It’s never bothered me a bit. I’ve probably even said it a time or two.
The one that really chafed my ass when I first started hearing it was “You’re good” in response to some decidedly minor bit of social awkwardness on my part after I apologized for it. Who are you to pronounce me “good,” asshole? But I’ve adjusted to it. No problem.
Heh. I’m old enough to remember all the kerfuffle about whether it was impertinent or otherwise inappropriate to tell people to “have a nice day”, which was a novel expression back then. “Don’t tell me what kind of day to have!”
Formulaic social phrases are going to keep evolving, and their new versions are going to keep on sounding weird to people who are unfamiliar to them and thus interpret them too literally. We’ve been sideswiped by the steamroller of linguistic evolution.
I’m 70 and ‘no problem’ doesn’t bother me at all.
I’m also not on social media. 
It sounded odd to me 25 years ago or so, when our colleagues in California would answer text messages (IMs) with “no problem”. For us in the upper Midwest, we just thought it was a quirky expression from a bunch of surfer dudes. But it was hard to NOT bring it into my own speech, because I knew that people in this part of the country would find it off-putting. Now, it doesn’t bother me (too much).
I can’t imagine getting fussed but this at all (I’m 56).
Honestly, I don’t think either would register much at all. The clerk could probably tell me to, “Go fuck yourself,” and I am likely to mumble a thanks and walk away. Even if I did notice I’d have to think if I cared enough to even get a manager and spend the time.
Often the best way to not let someone get satisfaction is to ignore them. Learned that when I was little and bullied.
These are literally “(it’s) nothing,” not “no problem,” though I suppose the connotation is similar.
I do sometimes hear pas de problème or kein Problem here in response to merci or danke, but it’s not common compared to the standard de rien or bitte (or gerne). I have no idea if the same generational irritation exists here as described in the OP.
(Luxembourgish being a mashup of surrounding languages, I also hear the “merci” / “kee Problem” combo, which is vaguely amusing.)
I’ve always figured if these stodgy old fussbudgets heard a British lady (think Hyacinth Bucket) say “why my dear, it’s no trouble at all,” they’d be thrilled. But let someone younger say “no problem,” which is really quite the same thing but with less words and no charming accent, and they seem to regard it as disrespect, anarchy, and impudence combined all wrapped up with an unspoken “and the horse you ride in on” to boot.
You haven’t earned mine, but that’s no problem. 
I can’t imagine getting fussed about it. Portuguese “you’re welcome” is also “de nada.”
There are at least two ways to respond to “thank you” in Indonesian, the most common probably being “sama sama” which translates as “the same to you.” But you can also say “tidak apa-apa” or assorted variations, all of which literally translate as “it’s nothing.”
I am skeptical that there are battalions of old people out there shaking their aged fists in rage because someone said “no problem” instead of “you’re welcome” to them. But if there are, they need to chill.
I’m Mr. Mole to you.
I’ll be 67 in a couple of days. Someone saying “no problem” instead of “you’re welcome” doesn’t bother me in the slightest.
Lately I’ve been hearing “have a good one”, which kind of grates on me a bit. It strikes me as lazily non-specific.
“No problem” doesn’t bother me, but it’s not a phrase I use, either. I’ve been known to say “no worries”, but that’s after someone has bumped into me and apologized, not after thanking me.
I’m pretty sure George Carlin did a bit about that in the '80s.
I think it was, “Have a nice day!”