I agree with what Emily said - and as I haven't read all of the posts I am not sure if I am repeating what others have said or not.
I think that when a child asks the question they are ready for the answer. I do think that each child is different though. Many people on the site were told at a young age the "truth" about CF and the likely outcomes. Many feel that is the best and only way to go. I personally do not. I do NOT think you should lie about it if your child approaches you, but I do not think it is necessary to have a sit down talk about it by the time your child reaches 1st grade or what have you.
I don't ever really remember asking or finding out about CF, but I know I did not find out all the details until I was about 7 years old or so. That really only happened because I had several friends with CF pass away. I never asked about it, but I figured it out on my own. I remember reading things in books as I got older - but I never really had that talk with my parents. I personally would not have been able to handle THAT talk at such a young age. I didn't want to hear those words from my parents - but that is me.
I will say any question I ever had my parents answered truthfully and if they did not know the answer they did their best to find out. I would not change the way I was raised in regards to how I found out about CF and all. I think it was right for ME. I think it just comes down to being truthful with your child if they ask you questions, don't hide anything, but remember that YOU will know your child best and YOU will know when is the right time to broach the topic and how to do so. You will know that better than anyone on this site. I just suggest do not hide or lie about any of it when questions do start to come about.
Take Care and Good Luck,
Lindsey
Editted to Add: Though I never had THAT talk and I wasn't really TOLD the truth ata young age - I still feel I turned out to be a relatively well adjusted adult anyway. So don't fear that if you don't tell your child be a cerain age they will be maladjusted.