I haven't really decided yet. Right now I'm leaning toward 15. Hopefully by then we have mapped out a vast majority of the sites she actually uses and it's not much of a transition.
I would think that is way too late unless she turns out to be an idiot without any interest in anything. You cannot reasonably research anything with a whitelist.
I disagree. Wikipedia has a lot of information.
Erm ... mind naming one hobby where Wikipedia has more than 1% of what there is to know about that hobby?
I imagine it's less about the number -- her age -- and more about her maturity and personality. I think your approach would work well, though she may need wider access to Google search results when doing homework.