Wife to Ben, husband extraordinaire! God is so good! 
The thread above will give you ideas about the challenges of CTC. Funny thing is he's fine with the work in Bigger now but I think he'd start to drown by CTC. Wife to Ben, husband extraordinaire! God is so good! 
My own sweet sister had her child doing CTC at age 9 and her second son is poised to hit CTC at age 9 as well. Mom2Monkeys wrote:You might find this thread helpful from those answering my questions about using CTC with my oldest this fall. viewtopic.php?f=6&t=10952
I personally do think it'd be a little much unless the 9yo did PHFHG as an 8yo, and then I would do CTC half speed AT FIRST. If the child didn't do PHFHG as an 8yo, I'd place in Bigger or Preparing.
So, are your kids coming up through the guides or from another program and starting HOD as a 9yo in CTC? I agree with Carrie, as I said in my other reply, that if they are coming up through the guides then you should be fine. But always know you can slow it down if need be. If you can get from these lower guides combined and up to CTC then one is ready to move forward and the other needs to slow down....that's doable too! Just split from there, taking one at full speed and the other at half. He'd grow into full speed sometime in there I'm sure.Carrie wrote:Katrina,
If your children are coming up through the HOD guides, and your current 7 year old fits best in Bigger and is able to do all that we are asking of him in Bigger... then it makes sense that he would be prepared to do Preparing at age 8 and CTC at age 9. So, it is possible for a 9 year old to do CTC (especially if that child came up through the HOD guides from Bigger on up).
Blessings,
Carrie
Just keep going and see what happens. If they are doing the guides they are in (if they are in HOD now) and are doing them well, placed correctly, you'll be just fine! My oldest will be 9.5 or almost 10 when she starts CTC. My second will possibly still be 8 when she starts, though turning 9 shortly after. My third will be 9 as well. I have no idea about my 4th. She's still so little that I cannot even start to tell how she'll do with a guide. But since they are all coming up through HOD I have no qualms about continuing in the next guide when we finish one, no matter what their ages. Riley will be 9 yo when he does it as well. I have found how they are doing with PHFHG to be a super indicator of how they will do with CTC. If your dc are doing well with everything in PHFHG by the close of that guide, they should do just fine with CTC. One thing to keep in mind is that dc are to grow into many skills in CTC. So, for example, in PHFHG dc learn to orally narrate well for the level of difficulty of reading in PHFHG throughout the year. In CTC, the oral narrations become harder, as the reading material becomes harder, but they are meant to grow into this level of difficulty and are not to be completely proficient at it at the start. We found the written narrations' instruction in PHFHG prepared my ds well for the written narrations at the start of CTC though. CTC starts off still holding their hand with this a bit, which I liked. I guess we found that there were enough familiar skills to balance out the newer skills that needed to be worked on all year. I just wanted to reassure you that we found CTC to be a wonderful fit for Wyatt when he was 9 yo, and from how PHFHG is going for Riley right now, I am sure it will be for him as well. Please ask more if I could be of any more help! . 
I have a 9 year old 3rd grader right now. This 3rd grader is using Bigger and he's a better reader and has MUCH better comprehension than his younger brother. I wouldn't want him reading The Story of the Ancient World on his own and I think the workload is more than he can handle at 9. My heart says "no" to asking my middle son to handle CTC as a young 9yo. Wife to Ben, husband extraordinaire! God is so good!