My son is six and I still brush his teeth. He does it first, then I go over it. He gets the practice while I get to make sure it's done correctly. I don't let him floss, I have to do that. He has a tendency to accidentally cut his gums. I do it, but teeth brushing is a chore I loathe.
My BIL is a dentist. He says that this issue is that children under about the age of 8 don't have the dexterity to really get it right, even if it looks like they are hitting all the right spots - apparently there are a lot of fine motor skills involved. My BIL also mentioned that rigorous studies have been done on the subject and when left to their own devices, kids under 8, even with parents watching, only removed about 25 % of the plaque on their teeth.
So I hate it, but I do it. It was a little bit of a fight for a while, but my son soon learned that he was going to have to suffer through me brushing his teeth, even though he constantly argued "I can do it". Now he just asks me if I want to brush before or after he does it.