Ren-
I am unaware of any government program for PG kids. I suppose if they existed, there wouldn't be as much need for a private organization like Davidson.
For the OP-
We let our kids set the pace and determine when they go away to school. Son number one went away at an age appropriate time with 50 college credits on his record. FWIW, many of the more competitive schools will use these classes as proof of a homeschoolers ability to perform, but they will not accept the credits. Son two (11) will be starting with a single college course in the next year, if we can get him in. He has a set of very competitive ACT scores, so I am hoping that we can avoid a real battle. He's thinking he may want to attend a service academy, which would mean that he would need to be at least 17.
A younger kid simply would not be up to the physical demands. We're keeping things open though, and trying to see where DS will really thrive.