Re: ages the curriculum is aimed at, I think we should ignore this. The curriculum should start from the ‘everyday’ ideas and finish at the point were the university curriculum starts.
If we were teaching adults we would not consider the age (you can’t do this until you’re 25, 40,60?) The main reason for grouping children by age is to organise their education at school – they will tend to be ready for concepts at a similar age and this is strongly reinforced, once they start school, by being in the same class so their experiences in school are the same. However, a child that moves school is often way ahead or way behind – which tells us that a lot of their current understanding is just due to their school experience. Some students could go much faster and some would benefit by taking things more slowly.
I don’t think this is relevant to the order they meet concepts, or the different routes through – which is what I think we should concentrate on. (And I hope this post doesn’t come out blank ….)