Jayden was 9 the entire time we've done it. Actually she might have started when she was 8, but not much... We started out going half speed for quite a while, like 10 units. Then we went two days half speed per week and one day full speed (5 day weeks). I'd say she really hit her stride with it at about 9.5.
She is the only one who has actually done it.... however looking down the road at my next two...
Grace would not be capable of doing it now. She just turned 8 in January. The readings would be too much, the independent work altogether would be to much. And the amount would be too much. Not to mention the content.
However, Julianna will be fully capable of it at 8. I'm not sure when she'll actually hit Preparing... what age she'll be. But if she's not doing it by the time she's 9 we might have to add in extensions for her. But I think that would be an odd occurance. I do not think she is a typical child.
I really can't tell with the other two yet
As far as their abilities... I think Jayden is very bright, but average where the work is concerned. The content would have been fine for her at newly 8, but the work involved would have been too much. Grace is beyond Jayden's abilities where comprehension and understanding is concerned, but she's more of a free spirit

So the actual amount of work is just too much. It's going to be too much when she gets to it, but she'll be 9 or close enough to it and I think it'll be time to buckle down a bit more. We'll go half speed as long as it takes. Julianna however is way above most kids her age in just about anything. She absolutely amazes me every single day with everything about her. THAT child will do great things. My others are going to do well in life... she's going to be a "great" person. It would not surprise me if she was being read ABOUT one day. This is why I think that if we don't hit preparing by the time she's 9, she'll need extensions. I just haven't done the numbers to see where she'll be farther down the road.
Overall I think Preparing is a perfect "4th grade" level. As written. I haven't used the extensions so I can't speak to that. I might change that after testing this year

I know Jayden has really grown a lot.... but since she's my oldest I"m not sure if it's the school/church stuff or just age.