Lots of good points here - I have one more comment. I'm a smallish (125 lb) female 1st kyu, but not so young (>35). When I started 6 years ago, in a different dojo than I currently am, I definitely experienced the patronizing or muscle man thing (mainly from ~3rd kyu guys), but when I moved to a different dojo, the senior students resisted the hell out of my technique but never ever patronized. And I'm glad they did, because even though it made being 5th and 4th kyu insanely frustrating, I can hold my own now. And, now that I have at least some skill, I find that big strong guys are sometimes very interested to know how I can throw them, being they've got 80-100 lbs on me and/or I'm older than them by 10 years.
a) how beginners, teens, old people and women are treated is to some extent dependent on the sensei and sempai of the dojo - the dojo culture, as it were.
b) just wait. the tables will turn if you stick it out long enough
c) I agree it's better in the long run to be underestimated than overestimated. You can catch people off-guard. Well, maybe. I'm blonde, and can get pretty annoyed w/ people who assume that means you have a low IQ (quite the opposite, in my case).