In a way, women in many places are underestimated by men and even other, older women.
Being younger in age also gives some the feeling that they know and understand more than you.
Much of the advice given in the posts before is very true.
Still, I think you will meet this behaviour alot more in the future. If you take it in a mature way (smiling and letting it pass, gently guideing the technique to what you want the other person to learn, showing confidence- but not OVER confidence, etc...), than you will see less and less of these behaviour towards you.
With no offence at all, I think that also, when you will be through of your teenage years, you will care less and less about it. most teenagers (and I'm not saying- you might be mature for your age), are more prone to radical mood swings and feelings, towards what has been said to them.
I know for me, things settled over time and I also started to take things in a lighter mood. I look very young for my age and get reactions in all works of life (I work in a hospital and already got comments such as why I'm not at school...). Sometimes when people tell me I look 16 (I'm really 27), I answer with a smile that I hope they'll say the same thing in 10 years time!
About the other Aikidoka ranking like you- I think they are very immature. After all, they must be training with you for quite some time and know your rank, yet they refuse to credit it. That's acting like children. It doesn't matter how young you are, every person can learn from anyone else, no matter younger or older, and we should respect each other for being human before anything else. I guess they missed some of the points of Aikido (the "spiritual" side).
I hope you find peace with dealing with these feelings. I know it can get frustrating sometimes, but accept the fact that you might meet these reactions in many other parts of your life and for many more years.
I say take it with a smile. You know who you are, and I guess your sensei also, and so do other sempais in your dojo. Enjoy their behaviour towards you, because it matters more. Hopefully, the others will learn from them with time.
Just my two blabering cents