In fact, that's what a lot of people like about college and it's why you see college described as a "place to experiment".
> their actual capabilities and weaknesses have also become clearer and more stable
Also not so sure about this. I've met MANY people who went in 99% sure of college major A but after a few classes and an internship they found out they suck at it/hate it.
Also, at that age, your ability to learn climbs very high just as function of age.
As long as the middle gets sorted into the Realschule, there's no problem ;-) But more seriously, according to wikipedia[0] IQ is relatively stable beginning at an age of approx. 11 years, so if we compensate for the measurement error with a permeable system I only see two problems: As you mentioned, at this age the brain devolops so rapidly that being the oldest in the class gives students a measurable advantage, but that is an issue for any school system, and as I said in a neighboring comment, overly ambitions parents are meddling too much, but that is a relatively recent phenomenon and a problem unto itself.
More to the point: you can't in one breath say "I know the difficulties surrounding IQ" and in the next say "any source I cite about IQ must represent settled science". One or the other of those statements must not be true.
[0] Deary, I. J., Whalley, L. J., Lemmon, H., Crawford, J. R., & Starr, J. M. (2000). The stability of individual differences in mental ability from childhood to old age: Follow-up of the 1932 Scottish mental survey. Intelligence, 28, 49 -55
There is a lot more then IQ that goes into grades. All of it happens before 10.
What would it add that the choice of university and major doesn't? The math kids at MIT aren't so different from each other.
At college age, people are much more capable of making their own decisions, and their actual capabilities and weaknesses have also become clearer and more stable. And it's not like going to a Fachhochschule is going to set someone up for a hard life as being assigned to a Hauptschule pretty much does.