At what age a child would develop a sense of racism or a minority discrimination?
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. 



Corenblum, B., &amp; Annis, R. C. (1993). Development of racial identity in minority and majority children: An affect discrepancy model. Canadian Journal of Behavioural Science/Revue canadienne des sciences du comportement, 25(4), 499.

Hirschfeld, L. A. (2008). Children’s developing conceptions of race. Handbook of race, racism, and the developing child, 37-54.

Katz, P. A., &amp; Kofkin, J. A. (1997). Race, gender, and young children. Developmental psychopathology: Perspectives on adjustment, risk, and disorder, 21, 51-74.

Pauker, K., Ambady, N., &amp; Apfelbaum, E. P. (2010). Race salience and essentialist thinking in racial stereotype development. Child Development, 81(6), 1799-1813.

Taylor, R. G. (1966). Racial stereotypes in young children. The Journal of Psychology, 64(2), 137-142.