At what age should children begin flossing? whenever they have teeth close together?

Age of flossing: I would suggest use of a waterpik and good brushing is even more effective than the use of floss and brushing. Either was, when the dentition is complete (not well related to age), that would be a good time.

Sure - even before: Since it's a good habit and many neglect to learn it, why not start as soon as possible - even if it doesn't do too much. Just to learn it as part of daily bathroom activity might be a good idea. I hope a dentist corrects me if i'm wrong. Sounds like you're doing good things with and for your child!

Never too early: As soon as a child can safely use the floss, it would be appropriate to do that. It is better to start good habits early.

Age 8 or 9: Teeth should be flossed as soon as they make contact with an adjacent tooth. To prevent decay a parent should floss & brush every evening for their child. A water pik (or better yet hydrofloss) can be used daily for healthy gums. Children do not have the physical dexterity until at least age 8 or 9 to properly and thoroughly clean their own teeth. Add Fluoride supplement if not in the waterr.