I seem to recall reading something written by Brian Lecomber on this recently - from memory (and don't quote me) it was along the lines of none of his students went solo below 15hrs as he taught them how to fly glide approaches and side slip before letting them loose. Very good article and thought provoking - think it was in Pilot or Flyer or FTN etc.
As said above, it is very much influenced by other factors beyond ability and I was much experienced at orbiting at the end of the downwind leg before going solo. It took me longer to solo (19hrs) that my entire CPL course (17hrs).