While I agree to much of your post, I have to argue about Piaget and language learning. Research has been done in that area, but it's not entirely conclusive when it comes to early vs. late language learning.
What is incontrovertible is that early learners learn all their languages in the same area of their brain, which is usually reserved for first language. Later learners (that is, after about 13 years old, but this varies according to sex and other individual characteristics) store other languages in another region of the brain.
It's not known whether this means that language stored in a different brain area is inherently weaker, however.
There are other major influences in language learning in the early and late levels, even strange things like ego: during your teens, you develop a sense of self and of belonging to a group (factors like accent are part of this process) and learning early can make a difference because there is no radical remaking of who you are (an English speaker changes to an English and French speaker, for example).
From my own experience, I have found that younger learners do well in some areas because of energy and enthusiasm, but older learners bring a much greater arsenal of cognitive assets to the classroom (things like memory), which you can tap into.
So pros and cons on both sides. Early advantage is a myth - and I'm a language teacher with years of experience in more than one country. I do agree that second languages should be learned as soon as possible, but that's from a systemic point of view: it's good for people to see similarities and differences between languages, and it helps you to view the world in a different way. But this should definitely not come at the expense of math, or science.
I'm all for a more rational approach to education, though, and in my opinion a huge part of the crisis in education comes from the fact that when formalized, whole-population schooling was adopted it was done not for the benefit of the children who were attending, but as a way of keeping the newly unemployed kids off the streets and out of trouble (this was during the industrial revolution but after the mechanization of the workplace, which took away a lot of the jobs that children did in factories).
It's because of this that school is structured by age rather than by ability. You should start at the same age, and then promotion should occur when you've shown you can handle the year's material.