How Early Should We Teach Philosophy?

I'm writing up a piece with the middle school teacher who invited me into her classroom for the last two years -- last year for a weekly logic/critical thinking module and this year's focusing on ethics and metaphysics. It has me thinking about philosophy in the classroom.

In the US, we don't teach philosophy until college. Some will have a philosophically minded English teacher who might slip some in and Catholic schools offer it earlier, but in the context of a specifically Catholic education. We always get excited first year students who had a taste and come back for more. It seems to say that high school students are certainly more than ready.

At the same time, it takes the brain a while before it can really process the abstract. The young mind is trying to figure out the concrete, so perhaps there is a too young to introduce philosophy. You need to have the standard way of thinking in place before questioning it becomes meaningful.

When would be the optimal age to begin teaching philosophy in schools. I understand that a study that encourages students and gives them the tools to effectively to question authority is not likely to end up in American classrooms anytime soon, but putting that aside, if we wanted to create maximally thoughtful, critical, and intellectual young citizens, at what age would we introduce them to philosophy?