OMG! i know EXACTLY how you feel! i have more stories than all the handfuls in my household (some are little hands, people) about situations people have mistook me for being in my teens, including at church AND on the phone! i even have a young sounding voice, i guess!
one of the places i've gotten it the worst is at the temple. just last week when i went to houston, one of the temple workers got brave and asked my age. i told her and her jaw dropped. she said all the other workers that saw me were discussing how i could not be old enough to be in there. she had defended me by saying i must be early twenties. she asked emphatically if it was genetic and wanted to see my parents to see if they looked young.
one time when i was in the temple supervising a youth trip for baptisms, the first counselor of the temple presidency came running, I am not exaggerating, he bolted down the front hall to demand that i was absolutely NOT allowed to be behind the desk! i patiently explained i was a leader.
even superman has to suffer at times. after q. was born, we decided to leave her for an hour and grab a quick bite for some quiet time. after ordering, the waitress came stomping back to our table and insisted i tell her my age. when i said, "i'm 25", she sighed and with a look of relief stated, to superman, "phew, i thought you were a pervert." OMG!!!!
perhaps laughing about all these silly experiences keeps us young. but i will tell you, having kids helps, just a little. i am getting more gray hairs!!!!
however, those could be from watching people look incredulous when i walk in a store with three or fours kids (depending on if i have a niece with me) and you can see it in their eyes, " someone needs to show that kid where to get birth control..."