Another CF person here...
I am 29 - and I hear the "you'll change your mind" comment repeatedly. Especially since I am still young and "capable". I absolutely think it is possible that I could change my mind one day, but am waiting for that bell to ring, and it simply hasn't yet. I question myself very deeply, and still find no desire for a child in my life.
Sometimes this is a hard thing for me to face for myself... The thought that when I am older I won't have that familial support network (Although I have lots of BFFs that could fill that role), and that my parents may never be grandparents... something they REALLY want. I think my mother has come to accept it though, since a discussion I had with her last year when I asked her to quit asking me about it.
Recently, I got a copper IUD that will ensure (99.8% anyhow) that I will remain child free until I am past forty. It is easily reversible for that small chance that I do change my mind.... this adds an incredible peace of mind in my decision, and I feel is the essence of feminism - the ability to choose.
I passionately support and respect those who do CHOOSE to have or not have children, and don't simply have them because they are supposed to, just like I passionately support those who CHOOSE their own relationship styles.