My response? "Well, I am a post doc at Fairly Big Institute and defended Y years back".
Next comment "oh, but you only look 18".
My slightly acidic response "I'm over 30".
And before someone says something, no I really don't think I look 18 and no, I don't find it charming or anything. I find it slightly condecending and if nothing else proves that someone wasn't really interesting in finding out what I was... although, I have to admit, I got a side comment "she really knows this stuff".
(bitter thoughts about looking young and trying to hack it in this world we call science.... but the conference over all is quite nice.)