that’s the thing, it doesn’t much matter when they start: someone with a 90 IQ could start in utero and they could not clear 2300. Not gonna happen.
But also think about the correlation thing: how many of those that start prepping in middle school continue on to junior/senior year? (Only the strong will survive.) Eventually, many of those average students will start earning average grades and the 'rents will recognize that elite college is out, so…