All good points, thanks. Everybody seems to be saying roughly similar things.
"You can't do catering or mechanics or even very practical subjects later in life without formal academic qualifications" Yes, there are typically entry requirements, but for practical subjects these are usually flexible and come down to demonstrating that you are capable of the work. If the young person discovers that he really does need, say, a maths GCSE and nothing else will do, there's no reason he can't do the GCSE at that stage. For the more academic careers where entry is going to be very competitive and the entry requirements quite inflexible, any young person who wants such a career will need to be highly self-motivated and academically-inclined anyway: if the child were the type of person who will only study if made to do so by somebody else, he's probably not cut out to be a vet.
Literacy. I hear stories of children who struggle hugely with literacy and are pushed to work on it and then manage to grasp it. I also hear of children who don't succeed with reading at school and for whom all the hard pushing seems to backfire, who feel like failures because so much of their time is being spent on something they can't do, who switch off to the possibility of learning because they have become convinced that they can't do it. Perhaps this prevents them from learning to read though they might have managed it if left alone. I know several dyslexic HE children who learned to read after the age of ten, having been very reluctant to read beforehand and not having been pushed to do so. People are ready for different things at different ages. I suppose it is impossible to say whether a particular child would have done better with a different approach to the one he had.
I don't know about windows of opportunity when it comes to physically disabled children getting stronger. Maybe it depends on the particular condition. Nobody has ever said to me that my own dd needs to get stronger RIGHT NOW or the opportunity to do so will be lost. She could certainly do with more exercise than she gets, and having more exercise does help her in many ways, that much is true. So I go to a lot of trouble to find activities and situations for her which she loves so much that she will voluntarily exercise. But I don't force her to exercise, aside from the fact that I'm increasingly unwilling to carry her short distances when I think she can manage to walk. She is getting stronger over time, undoubtedly more slowly than she would if I insisted on her doing exercise. But she is happy, there are no battles, and she seems to be getting there in her own time. Just as I don't care whether she learns to read at four or six or eight, I am not too bothered about the exact age at which she becomes able to walk a quarter mile.
I remember when she was younger, a physical therapist told me some ways I could work with dd to encourage her to learn to walk. These were all things which dd really disliked doing. I asked her, "If I don't do those things, will she still learn to walk someday?" She said yes. I asked whether it mattered if dd first walked at the age of two or three or four. She said no. "Then we only need to do these exercises if we want to hurry the process along?" She said yes. I told her that in that case I would leave dd to do it in her own time, and she said it was refreshing to work with a parent who wasn't in a hurry. In her experience parents aren't prepared to wait. They want their children to try to "catch up" with others of their age so they won't stand out so much at preschool and school. That was why she hadn't even mentioned to me that the exercises were optional.
Learning a language does seem to be something best done when a child is very young. But exposure alone should achieve this, so I don't see the need to require the child to work on it. If you have plenty of appealing books and TV programmes and computer games and songs in the target language, speak it sometimes and associate with native speakers when possible, then won't that be enough?
Just a few thoughts. I am ready to believe that there may be a few exceptional situations in which it is better to try to force a child to learn something, but in general I think they learn more effectively if the motivation comes from within.