I don't know a whole lot about juniors either, but it seems at least for men turning pro there aren't generally any junior eligible tour players of note. they all seem to take at least several years, between 18 and 21 to mature to have any sort of impact at bigger ATP tourneys. For women there's certainly been more teenage champions at the senior level, but even that seems to be a dying breed in today's power grunt/scream game.
the list of junior Wimby winners has a few big names scattered about, but it seems more often than not the player doesn't even become anything remotely recognizable:
http://en.wikipedia.org/wiki/List_of...ampions#Junior
it's certainly not like top draft picks in hockey that normally become at least NHL regulars. the Canadian sweep this year is certainly a bright spot for what recently seemed to be a flagging national program, but don't hold your breath for either one having a realistic shot in the big leagues.