'At what age do you think the advantage becomes really acute? I'm guessing A'level, which is the moment HE children sometimes seem to start going to college for one or more A levels, so maybe they are engaging directly with that issue. '
For sure at A level. Speaking from my own stand point of a biology teacher, it is a very rare child who can sort out the whole sylabus unaided. Even the very brightest, most well motivated students, need some help for at least some of the course. I've had to explan the dd bit to at least one , very highly thought of professor of psychology.
And remember that those people who deduced the subject material from the begining took years and years to do so! It would be a pointless exercise, in biological terms ,for a student to re-deduce the structure of DNA from the basic material, to start from scratch.
Good educators do facilitate, but sometimes we also have to explain.
Bright children would probably be able to cope with most of the GCSE science curriculum from textbooks. They wouldn't have as much fun, but they could probably manage.
Middle of the road kids would probably need some specialist help at GCSE, unless the parents had a grounding in the sciences.
Motivated , reasonably bright adults, could probably cope with the KS3 curriculum, but again the kids would miss out on stuff that parenst can't do at home, due to H and S contraints. Hard to explode potassium in the bath tub, for example!
They would also miss out on lots of group work.
In all cases having expert guidance makes learning easier, since teachers can see the pit falls, and understand a range of ways of getting the kids out of the hole they have found themselves in.