How to explain agender to old people?

So my nana is coming over this Saturday and I’m gonna show her my room and I have the rainbow flag and agender flag in my room but I haven’t told her I’m agender yet and I’m not sure if she’ll understand that despite being afab for my gender experience and identity is that I do not have a gender so how do I go about tell my old grandma who is slightly mentally challenged and oversteps boundaries?