Wow, I actually have one that no one else yet has mentioned.
"How old are you?"
Since I was around 14-15, I've always looked and sounded older than I am. One lady at a dentist office guessed that I was 25 when I was only 15. One kid (a rather sharp kid) thought about it for a minute before guessing that I was 30 when I was 18. I'm 24 NOW. Well, in a month, I'll be 25...
But even so, I've been around 6'1''/6'2'' (always forget) since 8th grade, and I've been able to grow a full, thick beard since I was 15, and my voice has been very deep since about 16, so people always think I'm older than I am. My friends used to joke that I could probably go buy alcohol and smokes from a shop and not get carded.
But then, around 21, my hair started to get really thin, and is now falling out in the 'vertex' part of my head in patches. I've tried to find out why, but it seems to be just bad luck. Not even my parents' hair has fallen out like mine has, and they're both over 60.
So, yeah, being a 24-year old using Rogaine, most people for the past few years assumed I was 28-30. 30 generally seems to be the age I look like, but I still sometimes get questions by people who just can't explain why I'm in positions that no 30-year old should be in. My only hope is that when I actually reach 30, I'll still look 30 when I'm older.