My mother-in-law is a grown up picky eater. She still eats like a child.
She’ll eat hamburgers, deli ham, white bread, orange juice, shrimp either breaded and deep fried or drenched in butter (ala Red Lobster’s Shrimp Scampi) but not prepared any other way. She eats most fast food, and drinks a lot of Pepsi.
She won’t eat fresh vegetables or salads. The only veggies I’ve seen her eat are canned corn and canned green beans.
It’s not just that she doesn’t like exotic or unusual food, or doesn’t like certain things - there’s a lot of mundane, plain food she won’t eat that I could never imagine an adult refusing to eat. She won’t eat bagels, or sub sandwiches, or any bread other that plain white bread and white hamburger buns. She eats chicken noodle soup, but no other kind of soup. She’ll eat things like roast beef with gravy, but not if you put any herbs or spices on it. It’s not that she prefers it one way or another - if you say “Oh, I put some fresh rosemary in with the roast” she won’t eat it.
She came to our Easter Brunch, where there was such exotic things as an egg bake thingy that consisted of eggs, crumbled breakfast sausage, and cheddar cheese, a broccoli and ham quiche, and bagels. She didn’t take even one bite of any of the food.
She’s a really sweet woman, always happy and nice to be around, but it’s really a challenge to have her over for dinner or take her out to eat. Hubby and I have talked about taking her on vacations - to see relatives, or a museum in a city, that kind of thing - and it always comes back to us going “What are we going to do for food? Eat Taco Bell 3 times a day for a week?”
At least with a picky kid, you can feed them something and the adults can eat something else. With a picky adult, all your food choices are determined by the picky eater. That’s fine every once in a while, but the hell if I’m going to spend several days eating crap because so-and-so refuses to eat anything at a real adult restaurant.