I don't know if it makes you feel better but you aren't alone. I was refused entry to a R rated movie when I was 21, I've had a census person ask me if my parents were home (I was in my 30's) and I've been carded at a restaurant with my parents when I was in my late 30's. Right now, "adulthood" still feels so new that it's bothering you. As you get older though, I think you'll come to appreciate it. Even now I have people refusing to believe I am as old as I say. And YES I know that doesn't help. People told me that all the time and I felt like "yeah right". So I get it.