I am interested in opinions, and if possible, references for published research, about the pros and cons of teaching abstract maths concepts to young children. My younger brother (five years old) understands negative numbers and square roots so I was thinking of trying to teach him about complex numbers and maybe some other concepts, but my elder brother (who is doing a maths/stats degree) said it was a crazy idea (without elaborating). Is such a question OK for this site ?
Your question seems fine. I think it would fit well under the tag (mathematics-education).
For at least 20 years Herman Rubin has been posting in various discussion groups his views of teaching mathematics to young children. Many have disagreed with his views, often pointing out that his own experiences growing up and that of his son are atypical examples. I think you will find some of these discussions of interest. See these sci.math posts, for example.
I do not see any problem in posting this question.
Also due to the higher level of understanding of your brother, it would represent a puzzle for him at his age to learn more complex math. If he is willing, i see no problem in teaching him. Furthermore if he presents interest why wouldn't you help him.