I agree, you should tell her now, not when she's older. For one thing, you don't want her to hear it from someone else (relative, neighbor kid, etc).
Sometime when she asks about her enzymes, treatments, etc. use those open doors to tell her tidbits about CF. Also, your attitide when you tell her (i.e. apologetic, sad, weepy, scared, anger, etc) may determine how she handles the news too.