Not saying my country school system is perfect, it isn't. And there are cases of abuse, bullying does exist. But 30 years ago I was the good, shy, student in a poor neighbourhood of a big city and never suffered or witnessed anything too bad (and from 4 to 12 year old we had a kid with Down syndrome in our class). And nobody would even think about splitting kids by ability, we don't have "advanced"/"honours" classes.
That split might not happen until age 18, but my wager is that it does happen everywhere. The question is when is the optimal time? I tend to think earlier rather than later.
I was not allowed into the gifted and talented program in my school because I had a lot of trouble with reading when I was young. However, I was probably the most gifted person in mathematics. But the G&T program was only for people who scored high on both.
So what happened? I was stuck in the average kids where I excelled but was bored. Anytime math was being worked, I was sent to the computer lab to play on the Apple IIes.
I eventually caught up from a reading perspective but the damage was already done. The american school systems aren't prepped to manage anything out of the ordinary. So since I wasn't in the gifted program, I didn't have the accelerated math during elementary school. Therefore, I couldn't start algebra as early. That means through out high school I was bored stupid. All because of what happened when I was 7 years old.
They made a decision on a 7 year olds ability.
Did it change my life for the worse? Probably not. Unintended consequences and all. I got super interested in computers since I was stuck in a computer lab all the time. I excelled in math in college. Majored in Computer Science with a minor in Mathematics.
BUT just because it worked out doesn't mean they didn't fuck up.
Now I have children. My oldest son ALSO did not qualify for G&T program. Can you guess why? He struggled with reading in early elementary school(we tried to help but sorry, people develop at different rates). Oh my gosh but he is good in math. The difference now is there is no escape for him like I had. He is stuck being bored in class.
My youngest son is adopted so we don't have much insight on family history from an academic standpoint but he is killing it in all aspects of school. He likely will be in the G&T program. Yet, I don't believe he is superior in intellect compared to my oldest. They have different skillsets.
So yeah, I think doing G&T determination in elementary school is pretty dang stupid. That crap will all fall into place naturally in middle school.
It is a system based on laziness and testing to the tests.
My wife volunteered a lot for the school and as a result she knew all the administration. She was able to get our preferred teacher each year etc. Get to know the teachers and administration, help them out. They can bend any rule at any time.
We are in private school now. In my son's 1st grade class (mostly 7 year olds), there is a kid who just turned 5. Private school could be a good option.
Public schools even within the same school system can be different, so dont generalize across the entire public school system.
What right do we have to impose these impacts on children? Or is the school tracking a form of trolley problem where it feels wrong to some to take an active choice?
Because you asked the direct question, I'll give a direct answer. "How early is earlier?" Based on an N of 5 (myself, my two siblings, and my two kids), I think it can be done productively as early as 3rd grade or so. (For me, it was done earlier than that [age 5], which I'm extremely happy about, but I'm not sure that's scalable or even typically appropriate.)
I do think that a periodic re-assessment is also appropriate. If someone "runs well" in 3rd grade and then reverts to the mean, they should revert to the mean academic leveling as well. If someone develops late and starts to excel in 7th grade, there should be a way for them to pivot towards the advanced classes then as well.