I get this too.  Just turned 29, people mistake me for 20 at most.  Where I work, there is a girl that is turning 20, and I asked my boyfriend how old he thought she was (because he comes with me sometimes and has seen her).  He said 'in her thirties at least'.  This chick is morbidly obese, 10 years younger than I am, and looks 10 years older. At least I get to feel really good about myself when people mistake me for being a decade younger than I am, ha ha.  Such a huge confidence boost!