I'm sure some of this has to do with me being very short, but I am mistaken for a lot younger than I am pretty often and it's usually in a way that does not feel like a compliment. It more comes across as "I know you are [X] age, not the age you are claiming to be, and therefore I don't respect you or trust you to do what you claim to be doing."
The first time I was harassed for not looking my age was when I was still in high school. My mom and I were grocery shopping together and she "celebrated" my turning 18 by asking me to go buy cigarettes for her at the customer service counter while she finished shopping for food. (In my area, you have to be 18 to buy tobacco products.) I did not want to do it but she wanted me to want to do it, if that makes sense. (She thought I would be so excited to buy cigarettes even though I was not into such things, and she was really disappointed that I wasn't itching to do Mature Things so she hollered at me in the store and "made" me do it.) So I went and did it, then waited for her to come to the grocery line. I gave her her bag of cigarettes and went and waited by the water fountains to let her finish checking out. And then store security came up and cornered me.
"Stay right there," they said. "Don't try to leave the store."
I was like "huh? I'm not trying to leave the store. Just waiting for my mom."
"AND WHERE ARE THE CIGARETTES??" the guy barked at me.
I told him I'd picked them up for my mom and she had them. They didn't want to let me walk over and show them where my mom was and kept arguing with me that I had done something nefarious with the cigarettes. I had to wait until she came over to figure out what the big deal was before they would let me go. Gosh, thanks for letting me do something that's So Mature, Mom!
The next time was when I went to Las Vegas for my birthday. (My dad had promised me this trip when I was four years old. He turned out to be a compulsive gambler later. He's in a program for it.) Obviously since I was newly 21 and knew very well that I looked young, I was expecting to get carded a lot in casinos. But at one point while sitting at a slot machine by myself in the middle of the night, a random security person walked up and asked me to show her my ID. I showed it and she said "You can't be here though. You aren't old enough."
Well that got my attention. "Huh? The ID shows my birthday. I'm old enough."
"NO YOU'RE NOT," she insisted. And said in a kind of threatening way that my birth date indicated I was under twenty-one. Considering that my dad and I had definitely planned this trip around my birthday so I would be allowed to do stuff, I surely was not mistaken about when it would be legal. I told her that was not the case, repeated what my birthday was and did the math for her, and she gave me a squinted look and told me she would be back, leaving and taking my ID card with her.
Shortly after, she came back, handed it back to me, and said "Happy birthday." And explained that she had been working overnights and didn't know what day it is anymore. Mmkay.
When I was in my mid-thirties, on the way to my mother's house, she asked me to pick up a few things at the store, including cigarettes. (This is a theme in my life.) The cashier asked for my ID to buy the cigarettes and at that point I laughed because if I had been HALF my age I would have still been old enough to buy the cigarettes. The cashier barked, "It ain't funny, honey."
I showed her my ID and she looked at it, looked at me, and then said "This isn't you."
"I can't let you buy these. This isn't you on the ID."
I told her I didn't know what she was talking about and yes, it was me and that was my ID. I'm guessing she was trying to process someone she'd thought was UNDER EIGHTEEN claiming to be born in the 1970s on their ID.
The woman told me that she was not going to put her job on the line by letting me "get away with" buying cigarettes, and she called her manager, preemptively voiding the sale. (I kept the receipt that I later got. It said "UNDERAGE CUSTOMER" on it.) I had to stand there and wait for her to check out other people in line while we waited for the manager to be available, which made me late to my mom's. Thankfully, when the manager finally did arrive, she just looked at my ID and said "Nah, that's her," and let me buy the cigarettes. The original cashier said absolutely nothing to me and went on checking out the other customers without even looking at me. Welp. At least the manager believed me and they didn't try to escalate.
Most recently, a man at my workplace told me I was using the term "bluescreen" incorrectly (when I said I had been kicked out of a meeting because my computer bluescreened). He insisted that bluescreens don't happen anymore (uh, yeah they do) and that the term referred to computers in the 1990s, when the error screen was "actually blue." (Yeah. Current bluescreens not only still exist but are still blue.) He claimed that I "would not remember" the original bluescreens because as a Millennial, this would predate me.
(Bluescreening computers are older than the 1990s, but let's not talk about that. Because I am also older than the 1990s.)
I informed him that I was not a Millennial and that I did remember the bluescreens of the 1990s because I dealt with them liberally when they were freezing me out of my college coursework.
"You were in college in the 1990s?" he asked. "I thought you were about 25 years old."
(So he thought I was more than TWENTY years younger than I was at the time.)
Other than that it's mostly been silly misunderstandings and goofy stuff. People ringing me up for kids' tickets at the movies even when we didn't ask for them, until I was in my mid-twenties. Being asked if I needed a booster seat or a kids' menu in a restaurant when I was in college. Having someone mistake me for my best friend's daughter in a bar (she's only 2 years older than me). Being turned away at a karaoke bar because they insisted I was underage, even though I was with someone who was younger than me whom they didn't even card. People finding out I'm old enough to be their mom when they thought I was their age. Multiple cases of buying alcohol (which I rarely do, but I have bought it for celebrations or for cooking/baking) and having people make weird/funny comments when they see my actual birth year.
I don't know if all of this is partly because everyone's supposed to try to look 20 until they're 70 so we have no idea what people in their 30s, 40s, and 50s look like, but damn is it annoying when disrespect is part of how younger people are treated and you never even get to grow out of it.