People thinking I'm younger than I actually am.
This is just an example of what I have to deal with. First, I am 19, almost 20. I look a bit younger than that and I guess I sound younger too according to this experience:
My dad and I were taking my younger brother to his orthodontist appointment. I called my brother's school to ask if they could send him down to the office, saving us a little time since my dad was running late. The woman on the phone was like, "Well you're going to have to get an adult to sign him out. We can't have children signing him out." or something like that. GRRRR! It makes me so mad!