At what age a child would develop a sense of racism or a minority discrimination? I wonder if what happened with my son(6yo) is a matter of concern or just a coincidence.

I think I need to give a bit of background to help analyze what happened.
We are an Arab family live in the UK, majority of my town is of European/Caucasian ethnicity, although there are few Arab families we know about, but we don't really communicate much.
He is the only student who speaks Arabic in his class (there are Polish and maybe other European languages), I don't know about the rest of the school, but if I guess, there isn't in this school.

Me and him were in the car, I was looking for a place to park, and just went by a car park but it wasn't available for normal parking (contract parking).
I said: We can't park here.
He said: Why? Is it because we speak Arabic?

At the beginning I laughed, and told him there is no such thing, but couple of seconds later I was wondering, how would he develop such idea?

I know this might sound like a troll, but I swear that what really happened, and I am concerned if my son is facing any acts of racism/discrimination, maybe in the school.
Children as young as two to three years old are able to pick up on racial categories and stereotypes, and to infer that others' behavior is driven by race (also gender and other categories).

Of course, at that age children are actively exploring causes in the world of all types, so a comment asking about Arabic language does not mean that a child is experiencing extensive discrimination due to race - on the other hand, I think it would be naive to think anyone with any minority characteristic is not experiencing any discrimination ever, especially in comments from other kids who are themselves learning about the world and other people. It also could have been something overheard on TV, it could have been from a discussion of discrimination rather than an experience, etc. 



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