When I was 17, I was working part time for my dad and I had to fly out of town to deliver some important papers. The secretary of the man I had to see took me out to lunch and asked if I wanted to order a drink. She was amazed when I told her my age - she thought I was at least 24. I’m pretty sure it was all in how I dressed. Plus I’m the oldest of 5 sibs, so I’m sure I looked worn and haggard… 
Since these conversations have devolved into anecdotage, I’ll post two of my own examples:
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I was mistaken for a doctor when I was 16. Granted, I was in a medical library doing research and had a prematurely receding hairline. On the other hand, I was wearing leggoons (remember those?) and a muscle shirt. Doctor walked up to me and asked my name. I gave it, and he said “Oh, Doctor _____!” I quickly fessed up, but the guy said I had him fooled.
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I was waiting for my girlfriend in a blues club (that served food, so minors were allowed). I went to the men’s room, and on my way back a young lady stepped in my way and gave me a “don’t you know me” type of look. It took me a second, then I realized it was my girlfriend’s daughter, who was 12-13 at the time. Honestly, she didn’t look at all out of place among the adults there. Had she wanted to pass for early 20s, she could have easily done so.
This makes me think that context is pretty key in most situations–you’re acting in the position of a secure courier, not many of whom are 17.
That means me. Once when I was 16 or 17, I was at the mall with my friend, and some grey-haired man in a suit standing outside of a jewelry store told me to come in, and look at around. I said no, that I couldn’t afford anything they were selling. He was super aggressive, but friendly, and told me just to look. He started pulling out all this fancy, way-too-much-money-for-my-poor-high-school self, at which point I assured him again that he was in no way going to get a sale out of me. He said that was fine, and began kibbitzing with me, asking me about my life and such, and at one point he asked me where I went to college. “I haven’t gone to college yet. I’m still in high school.” He looked at me puzzled and taken aback. “I’m 16,” I said, or however old I was.
He then abruptly put all the jewelry away and everything got weird.
Apparently he did not think was that young.
A guy in my car club said a friend of his in high school would shave his head to look like he had the doughnut from male pattern baldness so he could get beer on the weekends.
I’m 42 and have been carded for alcohol in the last month or so! Makes my DAY!
Wow, people under 9 years of age looking over 18? I’m not doubting that it could happen, just that I find it really surprising.
I think a lot of young runway models can look over 18, but in a different way than normal people looking over 18. I don’t know. Most of the people around me seem to have the opposite problem - looking like a minor when they are (way) older than 18.
Keep in mind that some people just respond with BS answers. 
I was 13 when I went to the county fair in 1980, and had the carnival guy guess my age. He guessed 18 because I was already growing facial hair.
You aren’t Hispanic, are you?
Growing up here in Phoenix, all the Mexican guys at school had mustaches before high school.
When I was 19, I looked 29. When I was 29, I looked 29. When I was 39, I looked 29.
Now I just look. 
Didn’t see if it has been mentioned, but Traci Lords managed to fool the adult film industry into thinking she was of age when she was actually 15 I believe. These people had a big interest in making sure their stars were of legal age and could have gotten into big trouble if they knowingly hired an underage person.
Keep in mind that the person who responded “10” is a medical doctor.
IIRC, almost student in my grade school class had at least one parent who’d immigrated from Mexico or Puerto Rico, if they hadn’t moved here at a young age themselves. A lot of the guys started getting facial hair well before we were out of eighth grade.
My brother Johnny & I both had full beards at 16 or so and had no trouble buying beer and cigs. Never got carded.
I had an extremely hot blonde friend in high school who was regularly mistaken for a good deal older. On more than one occasion I can think of, she bedded men who thought she was not jailbait.
I myself, around the age of 25 or 27—that range–very nearly committed statutory rape with a girl I met in a bar. I was saved from it only when I started singing a certain famous song from the 80s and and she did not recognize the song or the band, and I got suspicious. When I asked her how old she was she said 22, then laughed and admitted she was 15. :eek:
A medical doctor with poor eyesight?
I’m not buying that a 10 year old can look like an adult, but even if that were the case superficially, you would be able to tell after 5 minutes of contact that you were talking to a child.
Yeah, it’d have to be one magical ten year old.
Maybe Robin Williams?
On a related tangent:
Do race race/color, nationality, or gender seem to make a difference in general with people’s misconceptions?
Not meant to be inflammatory, genuinely curious.
My high school music teacher was right. One day karaoke will save me from a statutory rape charge. Between that and the math classes I am good to go for this lifetime.
I’m not too good at telling the ages of Thais.
I’m not kidding there. My sister in law is Thai, and I could not accurately tell her age or hte age of anyone in her family when I met them.