I've run into this. Usually I just try and explain that though my age/position/hobbies/whatever may lead them to expect X for me they can expect Y. If I say it in a friendly tone and am truly open to discussing the issue with the person I've found it goes okay. Sometimes people pout or make comments about how I'm so weird to not do X but I just smile and shake my head and keep on doing Y.

It's easier for me to follow through with the above suggestion once I've calmed down and thought about what made me frustrated to begin with. If I speak when my mind is full of thoughts on how they shouldn't stereotype and/or that they are annoying I don't usually communicate effectively and risk losing that person as a friend. I hope that helps.