I am in my 60s, had literally never heard of this prejudice til I was quite grown up, and it was a news item about bullying in school. took me a while to even understand what they were talking about. but I realise not this was unusual, though my older brother, 3 different schools, also had never come across it, and he went to a much larger, rougher school. and neither of us had such hair colour.
but obviously other people have/ do suffer because of these attitudes.
maybe the person who said it's the association with celts is right. remember the notices, no blacks/coloureds, no irish, no dogs.
we have to take note, and learn to be more civilised, and try to teach children to value people's character over their appearance.
I remember seeing tv of small children, aged about 4, friends, one black, one white. the interviewer was pushing them to say what the difference was between them. they answered in terms of likes, food, games, when pushed further the black boy exclaimed triumphantly, I know, Joshua lives at the bottom of the hill and I live at the top !