The entirety of the Holocaust itself is a picture of arguably one of the largest displays of racism in history. Keep in mind that Hitler used propaganda for years teaching Germans why they were the "superior race." Jews were not the only targets of the discrimination, though they were the largest group. Propaganda taught that anything that was not inherently Aryan was "less than" human. In Night, examples of discrimination start as Jewish families are moved from their homes and to the ghettos, then treated like animals as they are piled into train cars, and continues throughout the story as we see Elie's experience in the concentration camps.
In addition to racial discrimination, the first clear example of ageism and sexism comes at the very first selection in Section 3. As Elie, 14, and his father, 50, approach the front of the line, they are advised to lie about their ages. They tell the man they are 18 and 40. In this example of ageism, it is obvious that the selection means to weed out those who are too young or too old to work.
Also in this section, the women and children are split from the men. It is assumed that more women and children were killed during these initial selections for the same reason as above, they would have been less emotionally and physically fit to work.
Ageism is shown through the rest of the story in Elie's father's increasing fear of being chosen in each selection. He knows he is older than most, and considers it merely luck that he survives as long as he does.