From what I've seen, girls don't get the same encouragement that boys do. There are several "older" (older than I am) male guitarists in my usual music circle and they have offered me advice on occasion, but they seem to feel more comfortable with the boys than they do with me. Younger men either ignore me completely or raise their eyebrows when I set aside my dainty little lap dulcimer for the guitar. There are only a handful of guys who know me really well and have played with me for years that just treat me like any other musician.
I'm not a great guitarist but I still get the occasional "you're good for a girl". One music-store salesclerk asked me once if the strings I was buying were for my boyfriend (I shop elsewhere now). My guitar teacher always used to tell me to "stop playing like a girl" and get some VOLUME out of the guitar. I think I was his only female student at the time.
I don't usually seek out female musicians--I like the train songs, traveling songs, etc. that appear to be the domain mostly of male performers. This isn't to say I don't think females are as good--nobody beats Etta Baker--I just haven't found ones that "grab" me. I never liked the Indigo Girls, Ani DiFranco, etc.; I like Kate Wolf but some of her songs are just too . . . girly for me.