What age do you think a child should start flossing her teeth?

Earlier is better. When babies get their 16 teeth its a good time to attempt to genly floss their teeth. Most toddlers won't sit still but i would encourage you to make the attempt daily ( without fighting) as this will teach her an important habit which we do daily. Be persistent!
You do it now. Eye-hand coordination to floss usually not there until about age 10, but need to clean between teeth daily. Until your child can do it herself, have her sit in your lap facing away from you, head tilted back, and you floss her daily. You may also want to consider floss holders and/or water pik. Check w your dentist for suggestions and instructions.