The quintessential old person food in my mind is Hoarhound candy. I have no idea if old people actually like it or not, but damn if any time I’m around an old person there isn’t some Hoarhound candy in the vicinity. Nasty, vile stuff.
I loved liver and onions as a child and still do. Same for buttermilk. I can’t say I know very many people who like those things, though.
Old fart checking in here. You young wippersnappes don’t and won’t understand most of us geezers. Me, I eat bacon and eggs every morning. (I’m 65). I also eat pizza, Macdonalds hamburgers, and any spicey food I want to! Don’t make me barf or shit, though I do get a little heartburn at times.
Wait 'till you get there at my age. (and I hope you do!) You’ll probably do the same.
(Got the holidays ahead, hope you enjoy as much as I will!)
My Pucka (grandfather), always has a supply of hoarhound candy in his pocket. I associate the flavor with pocket lint and listening to the Red Sox on the radio. He was awesome. Crappy taste in sweets, though.
Semi-old person here.
I have always loved fruitcake. Of course, we always got it from the Collin Street Bakery which makes a dam fine fruitcake.
And mincemeat pie. Yummers. My mom’s was the best.
Ditto the Mallomars. And the rock candy.
Liver and onions, you can have 'em. Yuk.
My grandfather used to only chew Beeman’s gum. That’s a very old brand that never really made it into the modern age, though you can still buy it if you look around. (Clove and Black Jack were the other two old gum flavors.)
I just remember hating them all as a kid.
Elderberry wine. 
The good fruitcake was from Gethsemani Farms, the Trappist monastery in Kentucky, better known as the home of Thomas Merton. It included Kentucky bourbon. My very Catholic parents in Ohio got one of those fruitcakes every year. They sold well because they were moist and tasty.
Not that it’ll help, but. . .
Just because I’m curious to find out the answers to those questions as well.
Tripe.
brusselsprouts and borsht = old people food
Well, Jell-o salad was very popular in the Fifties. It was new and fast and space age. The whole jellied dessert thing was very posh before the invention of instant gelatin, so after it became widely available in instant form there was a bit of a craze. It’s certainly something I associate with old people.
My back-up sources are the Gallery of Regrettable Food and Nicola Humble’s Culinary Pleasures, a history of cookbooks in England.
Frozen dinners in tinfoil trays had the same kind of appeal. You could say that TV dinners haven’t become outdated so much as evolved, though, what with the tremendous market in prepared frozen food.
Wait, dried potato flakes probably fit, too. In the Fifties and early Sixties there was a huge move towards shelf-stable and frozen food. It was cool, new, scientific, and it had all these neat additives!
I have never disliked prunes, but I didn’t start eating them regularly until my mid-to-late-20s. I’m now almost 40, and I fully expect to continue eating prunes until I die blissfully in my sleep many decades from now.
I enjoy mincemeat pie but don’t go out of my way to get it. Those are about the only two things on your list that I like to eat.
My wild theory is that old people like those particular food items because they remind them of their loved ones who are now deceased. I mean come on, it’s been about a hundred and thirty years since horehound candy was popular. I figure several generations have been keeping it in circulation for the nostalgia value alone.
I wonder if younguns of the future will be wondering about Count Chocula, whether it’s always been an old person food or if old people just develop a taste for it.
Over 50 here (that’s all I’ll admit to, tyvm! :)). Postum was a coffee substitute during World War II - some folks actually liked the stuff (my Mom was one who did!), but many drank it only because they couldn’t get coffee then. Mincemeat pies have been around for a long time; they’re probably more popular in Europe than in the US, but I liked it as a kid and like it now, when I can get it, that is. Liver and onions I never liked, but years ago people ate it because they thought it was good for them. In defense of jell-o salads, I think they’re still popular - my family likes them and I usually make at least one over the holidays.
So maybe folks eat stuff because it reminds them of the good old days - they’re nostalgic. And some folks eat stuff just cause they like it. I don’t see any foods really fading out of popularity unless it’s stuff that medical science says isn’t healthy to eat - and even with that caveat, there’s a -lot- of foods that aren’t healthy which folks continue to indulge in. 
(Oh yeah, I’m talking about those fat-laden foods … unfortunately, it’s the fat that makes a lot of it taste so very good!).
JMHO, mind you.
Anyone catch the reference? (Hint: it isn’t about Elton John.)
Horehound candy was never intended to be a tasty snack confection. It was herbal medicine meant to bring relief to sore throats and bronchial coughs. Yes, it’s bitter and tastes nasty, which exactly its appeal when medicine is expected to taste bad. Nowadays we have Ricola which includes the herbal throat & cough relief, but tastes minty fresh.
Have you had your cholesterol checked lately? You might want to do that if you
expect to be around for a few more years. I’ve got a year or two on you, about four
years ago I woke up one morning and had trouble walking and talking, went to the
doc a couple days later and, after a CAT scan, discover that I’d had a stroke. I was
very lucky.
My grandma would eat anything she could get her hands on: tripe, pigeon, pigs feet, rabbit, cow heart etc. She grew up in a poor household and that sort of food was common amongst the poor when she was young, so, to answer the OP, it wasn’t something that she grew to like.
Old people tend to be cold. My grandmother (an extremely spry 93yo) was telling us last July that she had been getting worried one day because she had been feeling cold and “you know, that happened to X and Y and (insert here a whole alphabet of relatives) shortly before they died/their health went seriously down”, then she saw on TV that it was an unusually cold day and felt very relieved.
Also, there used to be a time when many of those meals weren’t seasonal, I know they weren’t when my gramps were growing up. My great-grandpa came from one of the poorest parts of the country, and they’d have “a slice of bread for breakfast, potatoes for lunch and bread soup for dinner” every day of the year except of feastdays; right now I’m in Costa Rica (talk about hot!) and many people eat rice and beans every day. Rice and beans for lunch, rice and beans for dinner. Occasionally, rice soup or mashed-up beans.
As far as I can tell, my mom ate the same kind of food her whole life, and so have I. (I’m 60.) Mom was born in 1922. She grew up poor, but they had “normal” food, the same stuff you’ll find on midwest family cafe menus today. I don’t know what constituted “popular” food in the 30’s and 40’s.
The only difference is they didn’t waste anything. If they butchered a pig, they pickled the feet. My grandma’s favorite part of fried chicken was the behind, and my mom loved the neck.
The only change was when she moved from Iowa to Seattle, where she developed a fondness for fresh seafood and pizza.
The only “old people” food I ever saw her eat was toast soaked in milk.
When I was growing up, she never served anything that I thought of as old-fashioned.
I’m still not sure that answers the OP.
Fruitcake will never disappear. 10,000 years from now, when the rest of our civilization is rubble, fruitcake will still be there, fresh as the day it was made. Unfortunately.
My grandfather liked Frozen Pudding Ice Cream from a young age. If anyone under age 60 eats it now, I’d be surprised. Actually, in regards to the OP, I would wonder if you can even find it sold anywhere anymore, outside of one or two small stands. My grandmother used to buy it in actual cartons from the grocery store. Yuck.