For what it's worth, most of my students solo at 15-20 hours, but I've signed them off as early as 10 and as much as 32. Personally, I soloed at 17 hours.
A lot depends on the standards the CFI holds the student to. I could probably teach someone to takeoff and land safely in good weather in less than 10 hours. I don't consider that the ultimate mark of being ready to solo though.
I want to see the "PIC attitude" in my students before they go on their own. I also want them to be able to handle at least 10 knot crosswinds...here in Nebraska winds can pick up quickly and unexpectedly. I don't want them to ball up a plane because they got caught in bad weather. I also make sure they can glide in and successfully put the plane in to a relatively small field well enough to walk away from in case of an engine failure. For a lot of students, that task alone might take 2-3 hours of training over nothing but emergencies. I don't ever want to get a call that one of my solo students got killed because he couldn't handle an engine failure on takeoff.
Turns around a point, steep turns, slow flight, etc., those things will all be perfected with time. It's pretty hard to wreck a plane doing slow flight. I'm more relaxed about those maneuvers. But directional control on the runway and being able to handle engine failures at any point in the flight, those items are absolutely critical in my book. I just don't see how anyone can become proficient to those standards in less than 10 hours.