It really varies from child to child. My oldest began losing teeth at 5, and now that she's 10, she only has 3 baby teeth left - the rest are her permanent teeth.
Now, my youngest is the polar opposite. She didn't lose her first baby tooth until she was 7. She will be 8 in June, and she has still only lost 2 baby teeth! Her top 2 front teeth are loose. One of them is literally being shoved out of the way by the adult tooth that is growing in under it, but that baby tooth is hanging in there so tight that we can't pull it out. We are starting to get worried we're going to have to have the dentist pull it!
One thing that I have noticed, though, is that my younger daughter has REALLY BIG adult teeth. Yes, she only has 2 of them so far, but they are huge. I'm wondering if maybe her baby teeth aren't coming loose, and/or the adult teeth aren't growing in, because her little mouth just isn't big enough to accommodate them. When she got a mouth expander/retainer, that's when the top 2 teeth finally became loose. It's almost like that extra little bit of space allowed them to start growing in, or something. I don't know.
Anyway, I guess it will happen eventually for all of them, right? :)